diff --git a/README.md b/README.md
index 8e700b7..3be302a 100644
--- a/README.md
+++ b/README.md
@@ -190,17 +190,24 @@ job is what keeps the car alive a while longer.
 Up to Phase 5 the world was scenery. Heat poured concrete on a road and that was
 the whole of the enemy. `sim/units.ts` is the layer that makes it inhabited.
 
-**Nothing appears any more — things are sent.** When a road turns notorious a
-patrol is *dispatched*: it starts at a junction a few hundred metres away, drives
-to the road, sweeps up and down it for about a minute, and while it is working
-the road it brings the heat back down. Then it leaves. The enemy is not punishing
+**Enemy ground is patrolled because it is enemy ground.** There are standing
+patrols on the roads wherever you are past the line, and more of them the deeper
+in you go — none in liberated territory, one in contested, three in occupied,
+five on the frontier. Nobody summoned these; they are what hostile territory
+looks like.
+
+**On top of that, nothing appears — things are sent.** When a road turns
+notorious a patrol is *dispatched*: it starts at a junction a few hundred metres
+away, drives there, sweeps up and down for about a minute, and while it works the
+road it brings the heat back down. Then it leaves. The enemy is not punishing
 you; they are re-securing a route and going home.
 
-**Checkpoints are built.** At `turret` an engineer is dispatched, and the tower
-rises over about forty seconds of someone standing on site. Arrive early and you
-find a half-built stump and no one on it. Once it stands, a gunner mans it, up on
-top so he shoots over his own barricade. Let the road cool and the whole thing is
-abandoned.
+**Everything on a road is built, including the concrete.** At `barricade` an
+engineer is dispatched and the blocks go up over about twenty seconds of someone
+standing there; at `turret` another is sent and the tower takes forty. Arrive
+mid-build and you find a half-finished stump with nobody on it. Only once the
+tower stands does a gunner man it, up on top so he shoots over his own barricade.
+Let the road cool and the whole thing is abandoned.
 
 **Traffic and people.** Civilian cars route across the network with somewhere to
 be; people wander on foot near the buildings. Both scatter when shooting starts.
diff --git a/src/heatProps.test.ts b/src/heatProps.test.ts
index fb859a5..39c6d27 100644
--- a/src/heatProps.test.ts
+++ b/src/heatProps.test.ts
@@ -2,7 +2,7 @@ import { describe, expect, it } from 'vitest';
 import * as THREE from 'three';
 import { createHeatProps } from './heatProps';
 import { createPhysics } from './physics/physics';
-import { createHeat, stepHeat } from './sim/heat';
+import { propsFor } from './sim/heat';
 import { generateWorld } from './sim/world';
 
 /**
@@ -10,62 +10,86 @@ import { generateWorld } from './sim/world';
  * so the prop lifecycle can be checked end to end — including that removing a
  * barricade removes its collider, not just its mesh.
  */
-describe('heat props lifecycle', () => {
-  it('builds and tears down colliders and meshes together', async () => {
-    const world = generateWorld(3, 0);
-    const physics = await createPhysics(world);
-    const scene = new THREE.Scene();
-    const props = createHeatProps(world.roads, physics, scene);
-    const heat = createHeat(world.roads);
+const world = generateWorld(3, 0);
+const segment = world.roads.segments[0]!;
+/** Somewhere the player is nowhere near the road in question. */
+const AWAY = { x: 99999, z: 99999 };
 
-    const baseBodies = physics.rapier.bodies.len();
-    const baseMeshes = scene.children.length;
+async function setup() {
+  const physics = await createPhysics(world);
+  const scene = new THREE.Scene();
+  const props = createHeatProps(world.roads, physics, scene);
+  return {
+    physics,
+    scene,
+    props,
+    baseBodies: physics.rapier.bodies.len(),
+    baseMeshes: scene.children.length,
+  };
+}
 
-    const drive = (metres: number) => {
-      for (let m = 0; m < metres; m++) {
-        props.sync(stepHeat(heat, { dt: 1 / 60, segmentId: 0, distance: 1 }), heat, 0);
-      }
-    };
-    const idle = (seconds: number) => {
-      for (let i = 0; i < seconds * 60; i++) {
-        props.sync(stepHeat(heat, { dt: 1 / 60, segmentId: null, distance: 0 }), heat, null);
-      }
-    };
-
-    drive(400);
-    expect(heat.level[0]).toBe('turret');
-    // Still on the road, so nothing has been built around the car yet.
+describe('barricades appear only once someone has built them', () => {
+  it('places nothing until a build is reported finished', async () => {
+    const { physics, scene, props, baseBodies, baseMeshes } = await setup();
+    // Heat alone does nothing now. An engineer has to have done the work.
+    for (let i = 0; i < 100; i++) props.sync(AWAY);
     expect(physics.rapier.bodies.len()).toBe(baseBodies);
+    expect(scene.children.length).toBe(baseMeshes);
 
-    // Leaving it lets the deferred work land.
-    idle(1);
-    const hotBodies = physics.rapier.bodies.len();
-    expect(hotBodies).toBeGreaterThan(baseBodies);
-    expect(scene.children.length - baseMeshes).toBe(hotBodies - baseBodies);
+    props.finished(segment.id);
+    props.sync(AWAY);
+    const expected = propsFor(segment, 'barricade').length;
+    expect(expected).toBeGreaterThan(0);
+    expect(physics.rapier.bodies.len() - baseBodies).toBe(expected);
+    expect(scene.children.length - baseMeshes).toBe(expected);
+  });
 
-    // Left alone, the road should give everything back.
-    idle(300);
-    expect(heat.level[0]).toBe('clear');
+  it('takes colliders and meshes away together when abandoned', async () => {
+    const { physics, scene, props, baseBodies, baseMeshes } = await setup();
+    props.finished(segment.id);
+    props.sync(AWAY);
+    expect(props.has(segment.id)).toBe(true);
+
+    props.abandon(segment.id);
+    expect(props.has(segment.id)).toBe(false);
     expect(physics.rapier.bodies.len()).toBe(baseBodies);
     expect(scene.children.length).toBe(baseMeshes);
   });
 
-  it('does not leak bodies when a road escalates through every level', async () => {
-    const world = generateWorld(3, 0);
-    const physics = await createPhysics(world);
-    const scene = new THREE.Scene();
-    const props = createHeatProps(world.roads, physics, scene);
-    const heat = createHeat(world.roads);
-    const baseBodies = physics.rapier.bodies.len();
+  it('waits rather than closing a slab around the car standing in the gap', async () => {
+    const { physics, props, baseBodies } = await setup();
+    const block = propsFor(segment, 'barricade')[0]!;
 
-    for (let cycle = 0; cycle < 3; cycle++) {
-      for (let m = 0; m < 400; m++) {
-        props.sync(stepHeat(heat, { dt: 1 / 60, segmentId: 0, distance: 1 }), heat, 0);
-      }
-      for (let i = 0; i < 300 * 60; i++) {
-        props.sync(stepHeat(heat, { dt: 1 / 60, segmentId: null, distance: 0 }), heat, null);
-      }
+    props.finished(segment.id);
+    // Parked right where the concrete is going.
+    props.sync({ x: block.x, z: block.z });
+    expect(physics.rapier.bodies.len()).toBe(baseBodies);
+
+    // Moved off; the build lands.
+    props.sync(AWAY);
+    expect(physics.rapier.bodies.len()).toBeGreaterThan(baseBodies);
+  });
+
+  it('does not leak bodies over repeated build and abandon cycles', async () => {
+    const { physics, scene, props, baseBodies, baseMeshes } = await setup();
+    for (let cycle = 0; cycle < 4; cycle++) {
+      props.finished(segment.id);
+      props.sync(AWAY);
+      expect(physics.rapier.bodies.len()).toBeGreaterThan(baseBodies);
+      props.abandon(segment.id);
       expect(physics.rapier.bodies.len()).toBe(baseBodies);
+      expect(scene.children.length).toBe(baseMeshes);
     }
   });
+
+  it('is idempotent, so a repeated report does not double up', async () => {
+    const { physics, props, baseBodies } = await setup();
+    props.finished(segment.id);
+    props.sync(AWAY);
+    const after = physics.rapier.bodies.len();
+    props.finished(segment.id);
+    props.sync(AWAY);
+    expect(physics.rapier.bodies.len()).toBe(after);
+    expect(after).toBeGreaterThan(baseBodies);
+  });
 });
diff --git a/src/heatProps.ts b/src/heatProps.ts
index 60c0f28..4bec13e 100644
--- a/src/heatProps.ts
+++ b/src/heatProps.ts
@@ -1,13 +1,17 @@
 /**
- * Turns heat levels into things that physically exist on the road.
+ * Puts the concrete on a road once someone has actually built it.
  *
- * This is an integration layer: it is allowed to touch both Rapier and three.js,
- * which is why it sits outside src/sim/. The sim decides *what* should be there
- * ({@link propsFor}); this only builds and tears it down.
+ * This used to be driven straight off the heat level: cross a threshold and a
+ * barricade existed. It does not any more. An engineer is dispatched, drives
+ * there, and stands on site while it goes up; only then does this get told to
+ * place anything. What arrives here is a finished build, not a state change.
+ *
+ * Integration layer: it is allowed to touch both Rapier and three.js, because it
+ * owns objects that must exist in both or in neither.
  */
 import * as THREE from 'three';
 import type RAPIER from '@dimforge/rapier3d-compat';
-import { propsFor, type HeatProp, type HeatState } from './sim/heat';
+import { propsFor, type HeatProp } from './sim/heat';
 import type { RoadNetwork } from './sim/roads';
 import type { PhysicsWorld } from './physics/physics';
 
@@ -17,19 +21,18 @@ const MATERIALS: Record<HeatProp['kind'], THREE.MeshStandardMaterial> = {
 
 const BOX = new THREE.BoxGeometry(1, 1, 1);
 
+/** Never close a barricade around the car that is standing in the gap. */
+const PLAYER_CLEARANCE = 8;
+
 interface Placed {
   bodies: RAPIER.RigidBody[];
   meshes: THREE.Mesh[];
 }
 
-export function createHeatProps(
-  roads: RoadNetwork,
-  physics: PhysicsWorld,
-  scene: THREE.Scene,
-) {
+export function createHeatProps(roads: RoadNetwork, physics: PhysicsWorld, scene: THREE.Scene) {
   const placed = new Map<number, Placed>();
-  /** Level changes waiting for the player to get off the road in question. */
-  const pending = new Set<number>();
+  /** Builds finished while the player was stood in the way, waiting for room. */
+  const waiting = new Set<number>();
 
   const clear = (segmentId: number) => {
     const existing = placed.get(segmentId);
@@ -39,9 +42,12 @@ export function createHeatProps(
     placed.delete(segmentId);
   };
 
-  const build = (segmentId: number, heat: HeatState) => {
+  const raise = (segmentId: number): void => {
+    if (placed.has(segmentId)) return;
     const segment = roads.segments[segmentId]!;
-    const props = propsFor(segment, heat.level[segmentId]!);
+    // 'barricade' is the level whose props are the concrete itself; the tower
+    // is a unit-owned structure and does not come from here.
+    const props = propsFor(segment, 'barricade');
     if (props.length === 0) return;
 
     const entry: Placed = { bodies: [], meshes: [] };
@@ -60,26 +66,39 @@ export function createHeatProps(
     placed.set(segmentId, entry);
   };
 
-  return {
-    /**
-     * Rebuild the segments whose level changed — but never the one the car is
-     * currently on. A barricade appearing around the player would spawn a static
-     * collider inside the chassis, and watching a checkpoint assemble itself in
-     * the mirror would break the fiction anyway. Changes wait until they are
-     * out of sight, which is how the player is meant to find them: by returning.
-     *
-     * Call every step, not only when something changed, so deferred work drains.
-     */
-    sync(changedSegmentIds: readonly number[], heat: HeatState, occupiedSegmentId: number | null) {
-      for (const id of changedSegmentIds) pending.add(id);
-      if (pending.size === 0) return;
+  const tooClose = (segmentId: number, player: { x: number; z: number }): boolean => {
+    const segment = roads.segments[segmentId]!;
+    return propsFor(segment, 'barricade').some(
+      (prop) => Math.hypot(prop.x - player.x, prop.z - player.z) < PLAYER_CLEARANCE,
+    );
+  };
 
-      for (const id of [...pending]) {
-        if (id === occupiedSegmentId) continue;
-        pending.delete(id);
-        clear(id);
-        build(id, heat);
+  return {
+    /** A barricade has been finished on this road. */
+    finished(segmentId: number) {
+      waiting.add(segmentId);
+    },
+
+    /** The road cooled off, or the checkpoint was abandoned. */
+    abandon(segmentId: number) {
+      waiting.delete(segmentId);
+      clear(segmentId);
+    },
+
+    /**
+     * Call every step. Places anything finished as soon as there is room — a
+     * block spawning on top of the car would put a static collider inside the
+     * chassis, so a player parked in the gap makes the last slab wait.
+     */
+    sync(player: { x: number; z: number }) {
+      if (waiting.size === 0) return;
+      for (const id of [...waiting]) {
+        if (tooClose(id, player)) continue;
+        waiting.delete(id);
+        raise(id);
       }
     },
+
+    has: (segmentId: number) => placed.has(segmentId),
   };
 }
diff --git a/src/main.ts b/src/main.ts
index 236ce46..9046872 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -217,15 +217,16 @@ async function boot() {
       { x: saved.car.angvel[0], y: saved.car.angvel[1], z: saved.car.angvel[2] },
       true,
     );
-    // Rebuild whatever the restored heat implies, so checkpoints are standing
-    // where the save says they are rather than appearing as you drive past.
     stepFront(front, 0);
     refreshSegmentControl();
-    heatProps.sync(
-      model.roads.segments.map((s) => s.id),
-      heat,
-      null,
-    );
+    // A save does not record who was standing where, so barricades on roads
+    // that were already hot are treated as already built rather than making the
+    // player wait for a fresh engineer to be sent out.
+    for (const segment of model.roads.segments) {
+      if (heat.level[segment.id] === 'barricade' || heat.level[segment.id] === 'turret') {
+        heatProps.finished(segment.id);
+      }
+    }
   }
 
   // Chassis transform at the end of the last two fixed steps, for interpolation.
@@ -296,15 +297,25 @@ async function boot() {
           // the ground the car happens to be standing on.
           decayFor: (id) => DECAY_MULTIPLIER[segmentControl[id]!],
       });
-      heatProps.sync(heatChanged, heat, currentSegment);
+      heatProps.sync({ x: at.x, z: at.z });
 
-      // Escalation is now something the enemy has to *do*. A road that turns
-      // notorious gets a patrol sent to it; one that turns worse gets an
-      // engineer sent to build a checkpoint. Both have to drive here first.
+      // Escalation is entirely something the enemy has to *do*. Every level
+      // sends somebody: a patrol to work the road, an engineer to pour the
+      // concrete, another to put the tower up. Nothing appears on its own.
       for (const id of heatChanged) {
+        const segment = model.roads.segments[id]!;
         const level = heat.level[id]!;
-        if (level === 'patrol') dispatchTo(units, model.roads, graph, model.roads.segments[id]!, 'patrol', chatterRng);
-        if (level === 'turret') dispatchTo(units, model.roads, graph, model.roads.segments[id]!, 'engineer', chatterRng);
+        if (level === 'patrol') {
+          dispatchTo(units, model.roads, graph, segment, 'patrol', chatterRng);
+        }
+        if (level === 'barricade') {
+          dispatchTo(units, model.roads, graph, segment, 'engineer', chatterRng, 'barricade');
+        }
+        if (level === 'turret') {
+          dispatchTo(units, model.roads, graph, segment, 'engineer', chatterRng, 'tower');
+        }
+        // Cooled back down: whatever is standing there stops being maintained.
+        if (level === 'clear' || level === 'patrol') heatProps.abandon(id);
       }
       // Driving a road is how you learn what is on it, and seeing ground is how
       // it stops being a blank on the map.
@@ -442,17 +453,23 @@ async function boot() {
         chatterRng,
       );
 
-      // Finished towers become solid; abandoned ones stop being solid.
-      for (const id of unitEvents.built) {
-        const site = units.checkpoints.get(id);
-        if (!site || towerBodies.has(id)) continue;
+      // Whatever an engineer just finished now exists in the world.
+      for (const { segment, stage } of unitEvents.built) {
+        if (stage === 'barricade') {
+          heatProps.finished(segment);
+          say('They have put concrete across that road.', 6);
+          continue;
+        }
+        const site = units.checkpoints.get(segment);
+        if (!site || towerBodies.has(segment)) continue;
         towerBodies.set(
-          id,
+          segment,
           physics.addStaticBox({ x: site.x, z: site.z, yaw: 0, width: 3, height: 6, depth: 3 }),
         );
         say('They have finished the tower on that road.', 6);
       }
       for (const id of unitEvents.removed) {
+        heatProps.abandon(id);
         const body = towerBodies.get(id);
         if (!body) continue;
         physics.removeBody(body);
diff --git a/src/render/units.ts b/src/render/units.ts
index 96139be..d1b2520 100644
--- a/src/render/units.ts
+++ b/src/render/units.ts
@@ -122,7 +122,10 @@ export function createUnitView(scene: THREE.Scene) {
       let towerCount = 0;
       for (const site of units.checkpoints.values()) {
         if (towerCount >= towers.count) break;
-        const built = site.built;
+        // A tower only stands once its own stage is done. While the barricade
+        // is going up there is nothing here but activity.
+        const built = site.done.includes('tower');
+        if (!built && site.stage !== 'tower') continue;
         const height = built ? 1 : Math.max(0.15, site.progress);
         position.set(site.x, (6 * height) / 2, site.z);
         scratch.compose(position, quaternion.identity(), scale.set(1, height, 1));
diff --git a/src/sim/units.test.ts b/src/sim/units.test.ts
index 91506b5..f9a045f 100644
--- a/src/sim/units.test.ts
+++ b/src/sim/units.test.ts
@@ -6,8 +6,10 @@ import { createFront, controlAt } from './regions';
 import { createHeat } from './heat';
 import { freshCondition } from './car';
 import {
+  AMBIENT_PATROLS,
   createUnits,
   dispatchTo,
+  hasBuilt,
   stepUnits,
   PATROL_HEAT_DECAY,
   patrolProgress,
@@ -25,7 +27,11 @@ function run(
   overrides: Partial<Parameters<typeof stepUnits>[1]> = {},
   rng = makeRng(3),
 ) {
-  const events = { built: [] as number[], removed: [] as number[], skirmish: false };
+  const events = {
+    built: [] as Array<{ segment: number; stage: string }>,
+    removed: [] as number[],
+    skirmish: false,
+  };
   for (let i = 0; i < seconds * 10; i++) {
     const step = stepUnits(
       state,
@@ -137,25 +143,47 @@ describe('patrols are dispatched, not conjured', () => {
   });
 });
 
-describe('checkpoints have to be built', () => {
+describe('everything on a road has to be built', () => {
   const segment = world.roads.segments[6]!;
+  const atSite = { player: { x: segment.ax, z: segment.az } };
 
-  it('does not exist until an engineer has stood there long enough', () => {
+  it('pours no concrete until an engineer has stood there long enough', () => {
     const state = createUnits();
-    dispatchTo(state, world.roads, graph, segment, 'engineer', makeRng(2));
-    // Nothing is standing yet.
+    dispatchTo(state, world.roads, graph, segment, 'engineer', makeRng(2), 'barricade');
+    // Dispatched, but nothing is standing on the road yet.
     expect(state.checkpoints.size).toBe(0);
 
-    const events = run(state, 200, { player: { x: segment.ax, z: segment.az } });
-    expect(events.built).toContain(segment.id);
-    const site = state.checkpoints.get(segment.id)!;
-    expect(site.built).toBe(true);
+    const events = run(state, 200, atSite);
+    expect(events.built).toContainEqual({ segment: segment.id, stage: 'barricade' });
+    expect(hasBuilt(state.checkpoints.get(segment.id), 'barricade')).toBe(true);
   });
 
-  it('is manned once it stands', () => {
+  it('does not raise a tower for a barricade job', () => {
     const state = createUnits();
-    dispatchTo(state, world.roads, graph, segment, 'engineer', makeRng(2));
-    run(state, 200, { player: { x: segment.ax, z: segment.az } });
+    dispatchTo(state, world.roads, graph, segment, 'engineer', makeRng(2), 'barricade');
+    run(state, 200, atSite);
+    expect(hasBuilt(state.checkpoints.get(segment.id), 'tower')).toBe(false);
+    // And nobody is manning a checkpoint that is only concrete.
+    expect(state.units.some((u) => u.role === 'garrison')).toBe(false);
+  });
+
+  it('takes a second engineer, and longer, to put the tower up', () => {
+    const state = createUnits();
+    dispatchTo(state, world.roads, graph, segment, 'engineer', makeRng(2), 'barricade');
+    run(state, 200, atSite);
+    state.dispatched.delete(segment.id);
+
+    dispatchTo(state, world.roads, graph, segment, 'engineer', makeRng(4), 'tower');
+    const events = run(state, 260, atSite);
+    expect(events.built).toContainEqual({ segment: segment.id, stage: 'tower' });
+    const site = state.checkpoints.get(segment.id)!;
+    expect(site.done).toEqual(['barricade', 'tower']);
+  });
+
+  it('mans the tower once it stands', () => {
+    const state = createUnits();
+    dispatchTo(state, world.roads, graph, segment, 'engineer', makeRng(2), 'tower');
+    run(state, 260, atSite);
     const gunner = state.units.find((u) => u.role === 'garrison');
     expect(gunner).toBeDefined();
     // Up on the tower, so it shoots over its own barricade.
@@ -164,20 +192,53 @@ describe('checkpoints have to be built', () => {
 
   it('is abandoned when the road stops being worth guarding', () => {
     const state = createUnits();
-    dispatchTo(state, world.roads, graph, segment, 'engineer', makeRng(2));
-    run(state, 200, { player: { x: segment.ax, z: segment.az } });
+    dispatchTo(state, world.roads, graph, segment, 'engineer', makeRng(2), 'tower');
+    run(state, 260, atSite);
     expect(state.checkpoints.size).toBe(1);
 
-    const events = run(state, 5, {
-      player: { x: segment.ax, z: segment.az },
-      heatLevel: () => 'clear',
-    });
+    const events = run(state, 5, { ...atSite, heatLevel: () => 'clear' });
     expect(events.removed).toContain(segment.id);
     expect(state.checkpoints.size).toBe(0);
     expect(state.units.some((u) => u.role === 'garrison')).toBe(false);
   });
 });
 
+describe('enemy ground is patrolled because it is enemy ground', () => {
+  /** Somewhere well past the line, so the player is in hostile territory. */
+  const deep = (() => {
+    const depth = front.boundaries.occupied + 60;
+    return { x: front.axis.x * depth, z: front.axis.z * depth };
+  })();
+
+  it('keeps no patrols behind your own lines', () => {
+    const state = createUnits();
+    run(state, 300, { player: { x: world.spawn.x, z: world.spawn.z } });
+    expect(state.units.some((u) => u.role === 'patrol')).toBe(false);
+  });
+
+  it('has patrols already working the roads out past the front', () => {
+    const state = createUnits();
+    run(state, 200, { player: deep });
+    const patrols = state.units.filter((u) => u.role === 'patrol');
+    // Nobody triggered these: the player never drove anything here.
+    expect(patrols.length).toBeGreaterThan(0);
+    for (const patrol of patrols) expect(patrol.assigned).not.toBeNull();
+  });
+
+  it('puts more of them on more dangerous ground', () => {
+    expect(AMBIENT_PATROLS.liberated).toBe(0);
+    expect(AMBIENT_PATROLS.contested).toBeLessThan(AMBIENT_PATROLS.occupied);
+    expect(AMBIENT_PATROLS.occupied).toBeLessThan(AMBIENT_PATROLS.frontier);
+  });
+
+  it('never stacks two patrols onto the same road', () => {
+    const state = createUnits();
+    run(state, 400, { player: deep });
+    const roads = state.units.filter((u) => u.role === 'patrol').map((u) => u.assigned);
+    expect(new Set(roads).size).toBe(roads.length);
+  });
+});
+
 describe('a war going on regardless', () => {
   it('breaks out fights between the two armies', () => {
     const state = createUnits();
diff --git a/src/sim/units.ts b/src/sim/units.ts
index 1c6f908..c135655 100644
--- a/src/sim/units.ts
+++ b/src/sim/units.ts
@@ -55,17 +55,34 @@ export interface Unit {
   cooldown: number;
   /** Height of the muzzle, so a gunner on a tower shoots over the barricade. */
   elevation: number;
+  /** What an engineer was sent to put up. */
+  building?: BuildStage;
 }
 
+/**
+ * What can be put up on a road, in the order it goes up.
+ *
+ * Both stages are *work*. Concrete does not pour itself the instant a road
+ * becomes notorious any more than a tower does — someone has to be sent, arrive,
+ * and stand there while it goes up.
+ */
+export type BuildStage = 'barricade' | 'tower';
+
 export interface Checkpoint {
   segment: number;
-  /** 0..1. A tower does not exist until someone builds it. */
+  /** What is going up right now. */
+  stage: BuildStage;
+  /** 0..1 toward the current stage. */
   progress: number;
-  built: boolean;
+  /** Stages finished and standing. */
+  done: BuildStage[];
   x: number;
   z: number;
 }
 
+export const hasBuilt = (site: Checkpoint | undefined, stage: BuildStage): boolean =>
+  site?.done.includes(stage) ?? false;
+
 export interface UnitState {
   units: Unit[];
   checkpoints: Map<number, Checkpoint>;
@@ -98,11 +115,30 @@ const SPEED: Record<UnitKind, number> = { car: 14, soldier: 2.4 };
 const PATROL_DURATION = 90;
 /** Heat removed per second by a patrol actually driving its assigned road. */
 export const PATROL_HEAT_DECAY = 0.012;
-/** Seconds of an engineer standing on site to finish a checkpoint. */
-const BUILD_SECONDS = 40;
+/**
+ * Seconds of an engineer standing on site per stage. Concrete is quicker than a
+ * tower, but neither is instant — arriving mid-build is meant to be a thing that
+ * happens to you.
+ */
+const BUILD_SECONDS: Record<BuildStage, number> = { barricade: 22, tower: 40 };
 /** Minimum gap between skirmishes breaking out. */
 const SKIRMISH_SPACING = 55;
 
+/**
+ * Standing patrols the enemy keeps on the roads near the player, by whose
+ * ground it is.
+ *
+ * These are not a reaction to anything the player did. Occupied territory is
+ * patrolled because it is occupied, and the deeper in you go the more of it
+ * there is. Heat-triggered patrols arrive on top of this.
+ */
+export const AMBIENT_PATROLS: Record<Control, number> = {
+  liberated: 0,
+  contested: 1,
+  occupied: 3,
+  frontier: 5,
+};
+
 export const UNIT_HP: Record<UnitKind, number> = { car: 60, soldier: 30 };
 
 // --- Helpers --------------------------------------------------------------
@@ -154,8 +190,18 @@ function advance(unit: Unit, roads: RoadNetwork, dt: number): boolean {
   return false;
 }
 
+/**
+ * Node list for a journey, *including* the node it starts from.
+ *
+ * Dropping the start node looked harmless and was not. A unit re-routing from
+ * mid-segment is not standing on the node its route begins at, so its first leg
+ * became a straight line to a node two hops away — across buildings, across
+ * open country, wherever that line happened to go. Keeping the start node makes
+ * a unit rejoin the road before setting off. Anything already standing on it
+ * shifts past within a step.
+ */
 function routeTo(graph: Graph, roads: RoadNetwork, from: number, to: number): number[] {
-  return findRoute(graph, from, to, travelTime(roads))?.nodes.slice(1) ?? [];
+  return findRoute(graph, from, to, travelTime(roads))?.nodes ?? [];
 }
 
 // --- Spawning -------------------------------------------------------------
@@ -234,6 +280,7 @@ export function dispatchTo(
   segment: RoadSegment,
   role: 'patrol' | 'engineer',
   rng: Rng,
+  stage: BuildStage = 'barricade',
 ): Unit | null {
   if (state.dispatched.has(segment.id)) return null;
 
@@ -241,7 +288,8 @@ export function dispatchTo(
   // the patrol arrives from somewhere plausible rather than materialising.
   const origins = roads.nodes.filter((n) => {
     const d = Math.hypot(n.x - segment.ax, n.z - segment.az);
-    return d > 150 && d < 600;
+    // Far enough to be somewhere else, close enough to arrive while it matters.
+    return d > 150 && d < 430;
   });
   if (origins.length === 0) return null;
   const origin = origins[Math.floor(rng() * origins.length)]!;
@@ -265,9 +313,57 @@ export function dispatchTo(
     onStation: 0,
     cooldown: 1,
     elevation: 1,
+    building: role === 'engineer' ? stage : undefined,
   });
 }
 
+/**
+ * A patrol that is simply *there*, already working a road in enemy territory,
+ * rather than one summoned by the player's own tracks.
+ *
+ * Spawned onto its road rather than driven in from a depot: hostile ground
+ * should have patrols on it the moment you arrive, not thirty seconds later.
+ */
+function spawnAmbientPatrol(
+  state: UnitState,
+  roads: RoadNetwork,
+  near: { x: number; z: number },
+  front: Front,
+  rng: Rng,
+): boolean {
+  const candidates = roads.segments.filter((segment) => {
+    if (state.dispatched.has(segment.id)) return false;
+    const mid = { x: (segment.ax + segment.bx) / 2, z: (segment.az + segment.bz) / 2 };
+    const gap = distance(mid, near);
+    // Not on top of the player, not beyond what is simulated.
+    if (gap < 90 || gap > SIM_RADIUS) return false;
+    return controlAt(front, mid.x, mid.z) !== 'liberated';
+  });
+  if (candidates.length === 0) return false;
+
+  const segment = candidates[Math.floor(rng() * candidates.length)]!;
+  const start = pointOnSegment(segment, rng(), 0);
+  state.dispatched.add(segment.id);
+  makeUnit(state, {
+    kind: 'car',
+    faction: 'enemy',
+    role: 'patrol',
+    x: start.x,
+    z: start.z,
+    heading: start.yaw,
+    speed: SPEED.car,
+    hp: UNIT_HP.car,
+    path: [],
+    expires: 260 + rng() * 200,
+    assigned: segment.id,
+    // Start part-way through a sweep, so they are not all in step.
+    onStation: rng() * 20,
+    cooldown: 1,
+    elevation: 1,
+  });
+  return true;
+}
+
 /** Two squads run into each other. The player is not invited. */
 function spawnSkirmish(
   state: UnitState,
@@ -330,9 +426,9 @@ export interface UnitStep {
 }
 
 export interface UnitEvents {
-  /** Checkpoints finished this step, so the renderer can build them. */
-  built: number[];
-  /** Checkpoints abandoned, so the renderer can take them away. */
+  /** Stages finished this step, so the world can put them up. */
+  built: Array<{ segment: number; stage: BuildStage }>;
+  /** Checkpoints abandoned, so the world can take them away. */
   removed: number[];
   skirmish: boolean;
 }
@@ -357,6 +453,12 @@ export function stepUnits(
     spawnPedestrian(state, player, rng);
   }
 
+  // --- Enemy ground is patrolled because it is enemy ground ---
+  const patrols = state.units.filter((u) => u.role === 'patrol').length;
+  if (patrols < AMBIENT_PATROLS[controlAt(step.front, player.x, player.z)] && rng() < dt * 1.5) {
+    spawnAmbientPatrol(state, roads, player, step.front, rng);
+  }
+
   // --- A war going on regardless of the player ---
   if (step.now - state.lastSkirmishAt > SKIRMISH_SPACING && rng() < dt * 0.5) {
     if (spawnSkirmish(state, roads, player, step.front, rng)) {
@@ -419,20 +521,32 @@ export function stepUnits(
           step.decayHeat(segment.id, PATROL_HEAT_DECAY * dt);
           if (unit.onStation > PATROL_DURATION) unit.expires = 0;
         } else {
+          const stage: BuildStage = unit.building ?? 'barricade';
           const site = state.checkpoints.get(segment.id) ?? {
             segment: segment.id,
+            stage,
             progress: 0,
-            built: false,
+            done: [],
             x: point.x,
             z: point.z,
           };
           state.checkpoints.set(segment.id, site);
-          if (!site.built) {
-            site.progress = Math.min(1, site.progress + dt / BUILD_SECONDS);
-            if (site.progress >= 1) {
-              site.built = true;
-              events.built.push(segment.id);
-              // Someone has to man it once it stands.
+
+          if (site.done.includes(stage)) {
+            // Already standing; nothing left for this one to do.
+            unit.expires = 0;
+            break;
+          }
+
+          site.stage = stage;
+          site.progress = Math.min(1, site.progress + dt / BUILD_SECONDS[stage]);
+          if (site.progress >= 1) {
+            site.done.push(stage);
+            site.progress = 0;
+            events.built.push({ segment: segment.id, stage });
+
+            // A tower is no use unless someone is standing on it.
+            if (stage === 'tower') {
               const post = pointOnSegment(segment, 0.5, segment.width / 2 + 2.5);
               makeUnit(state, {
                 kind: 'soldier',
@@ -451,8 +565,8 @@ export function stepUnits(
                 // Up on the tower, shooting over its own barricade.
                 elevation: 5,
               });
-              unit.expires = 0;
             }
+            unit.expires = 0;
           }
         }
         break;
@@ -503,7 +617,11 @@ export function stepUnits(
   // --- Retire the dead, the finished and the far away ---
   const survivors: Unit[] = [];
   for (const unit of state.units) {
-    const tooFar = distance(unit, player) > SIM_RADIUS * 1.3;
+    // Anything with a job keeps it regardless of distance. Culling by range
+    // caught units that were dispatched from further out than the cull radius
+    // and deleted them on their first step — so the patrol or the engineer the
+    // player had earned simply never turned up. Their `expires` bounds them.
+    const tooFar = unit.assigned === null && distance(unit, player) > SIM_RADIUS * 1.3;
     if (unit.hp <= 0 || unit.expires <= 0 || tooFar) {
       if (unit.assigned !== null && (unit.role === 'patrol' || unit.role === 'engineer')) {
         state.dispatched.delete(unit.assigned);