diff --git a/game/README.md b/game/README.md
index 90e078e..9b6bbc3 100644
--- a/game/README.md
+++ b/game/README.md
@@ -51,4 +51,7 @@ Space/K or gamepad A/Cross jumps, with about 52px tap height and 96px held heigh
 0.1s coyote time and 0.12s buffering. Landing uses the drawn landing pose; falls
 of at least 152px use the four-frame hard landing. Authored walk polylines follow
 slopes through 40°; steeper surfaces accelerate downhill, with jumping still available.
-F2 shows the contact slope and slide state. One-way platforms and ladders follow separately.
+F2 shows the contact slope and slide state. Yellow one-way decks catch falling feet;
+Down + Jump drops through them. W/S, up/down arrows or gamepad up/down enter and
+move along blue climb zones at 70px/s. Release to hold, move sideways or jump to
+leave. `Player.canAttack` is false while climbing. Reserved ledge points stay inactive.
diff --git a/game/src/player/body.ts b/game/src/player/body.ts
index 827190c..3a30c7d 100644
--- a/game/src/player/body.ts
+++ b/game/src/player/body.ts
@@ -9,7 +9,7 @@ export function overlaps(body: Body, box: Box) {
 }
 
 /** Feet origin and continuous polylines; one-way policy follows separately. */
-export function stepBody(body: Body, geometry: Geometry[], dt: number) {
+export function stepBody(body: Body, geometry: Geometry[], dt: number, ignoreOneWay = false) {
   const oldX = body.x, oldY = body.y;
   const wasGrounded = body.grounded;
   const oldSlope = body.slope ?? 0;
@@ -33,7 +33,7 @@ export function stepBody(body: Body, geometry: Geometry[], dt: number) {
   body.sliding = false;
   body.slope = 0;
   for (const world of geometry) {
-    for (const line of [...world.walk, ...world.slide]) for (let i = 1; i < line.length; i++) {
+    for (const line of [...world.walk, ...world.slide, ...(ignoreOneWay ? [] : world.oneWay)]) for (let i = 1; i < line.length; i++) {
       const [a, b] = [line[i - 1], line[i]];
       if (body.x < a[0] || body.x > b[0]) continue;
       const slope = (b[1] - a[1]) / (b[0] - a[0]);
diff --git a/game/src/player/player.ts b/game/src/player/player.ts
index c6abb0a..eafe85a 100644
--- a/game/src/player/player.ts
+++ b/game/src/player/player.ts
@@ -3,6 +3,7 @@ import type { CollisionChunk } from '../stage/collision';
 import { BODY_HEIGHT, BODY_WIDTH, overlaps, stepBody, type Body } from './body';
 import { horizontal, move, movement } from './movement';
 import { jump, jumpState, landing } from './jump';
+import { climb, dropThrough, traversal } from './traversal';
 
 export class Player {
   readonly body: Body;
@@ -15,12 +16,14 @@ export class Player {
   private lastSpeed = 0;
   private spawn;
   private air;
+  private traversal = traversal();
+  get canAttack() { return this.traversal.ladder === null; }
   constructor(private scene: Phaser.Scene, private chunks: CollisionChunk[]) {
     this.spawn = chunks.flatMap(c => c.world.spawn).find(p => p.kind === 'player') ?? { x: 120, y: 461 };
     this.body = { x: this.spawn.x, y: this.spawn.y, vx: 0, vy: 0, grounded: true };
     this.air = jumpState(this.body.y);
     this.sprite = scene.add.sprite(this.body.x, this.body.y, 'hero-move', 'idle-00').setOrigin(0.5, 1).setDepth(100);
-    this.keys = scene.input.keyboard!.addKeys('A,D,LEFT,RIGHT,SPACE,K') as Record<string, Phaser.Input.Keyboard.Key>;
+    this.keys = scene.input.keyboard!.addKeys('A,D,W,S,LEFT,RIGHT,UP,DOWN,SPACE,K') as Record<string, Phaser.Input.Keyboard.Key>;
     this.debug = scene.add.graphics().setDepth(10001);
     this.label = scene.add.text(20, 80, '', { fontFamily: 'monospace', fontSize: '18px', backgroundColor: '#17242d' })
       .setScrollFactor(0).setDepth(10002).setVisible(false);
@@ -33,15 +36,26 @@ export class Player {
     const axis = horizontal(this.keys.A.isDown || this.keys.LEFT.isDown, this.keys.D.isDown || this.keys.RIGHT.isDown,
       pad?.leftStick.x, pad?.left, pad?.right);
     const jumpDown = this.keys.SPACE.isDown || this.keys.K.isDown || !!pad?.A;
+    const vertical = horizontal(this.keys.W.isDown || this.keys.UP.isDown, this.keys.S.isDown || this.keys.DOWN.isDown,
+      pad?.leftStick.y, pad?.up, pad?.down);
+    const geometry = this.chunks.map(c => c.world);
     // Substeps keep wall and floor contacts stable during slow rendered frames.
     let remaining = Math.min(milliseconds / 1000, 0.1);
     while (remaining > 1e-9) {
       const dt = Math.min(remaining, 1 / 120);
-      move(this.motion, axis, dt);
-      if (!this.body.sliding) this.body.vx = this.motion.vx;
-      jump(this.body, this.air, jumpDown, dt);
+      const pressed = jumpDown && !this.air.wasDown;
+      const dropped = vertical > 0 && pressed && dropThrough(this.body, this.traversal, geometry);
+      const climbing = climb(this.body, this.traversal, geometry, vertical, axis, pressed, dt);
       const wasGrounded = this.body.grounded;
-      stepBody(this.body, this.chunks.map(c => c.world), dt);
+      if (dropped || climbing) {
+        this.air.wasDown = jumpDown; this.air.buffer = 0; this.air.coyote = 0; this.air.active = false;
+        this.motion = movement();
+      } else {
+        move(this.motion, axis, dt);
+        if (!this.body.sliding) this.body.vx = this.motion.vx;
+        jump(this.body, this.air, jumpDown, dt);
+      }
+      if (!climbing) stepBody(this.body, geometry, dt, this.traversal.drop > 0);
       landing(this.body, this.air, wasGrounded, dt);
       this.motion.vx = this.body.vx;
       remaining -= dt;
@@ -50,8 +64,10 @@ export class Player {
       Object.assign(this.body, { x: this.spawn.x, y: this.spawn.y, vx: 0, vy: 0, grounded: true, sliding: false, slope: 0 });
       this.motion = movement();
       this.air = jumpState(this.body.y);
+      this.traversal = traversal();
     }
-    const pose = this.air.landing > 0 ? this.air.hard
+    const pose = this.traversal.ladder ? `ladder-${String(Math.floor(this.body.y / 13) % 4).padStart(2, '0')}`
+      : this.air.landing > 0 ? this.air.hard
       ? `hard-landing-${String(Math.min(3, Math.floor((4 / 12 - this.air.landing) * 12))).padStart(2, '0')}`
       : 'jump-08' : this.body.sliding ? `fall-${String(Math.floor(this.scene.time.now * 12 / 1000) % 3).padStart(2, '0')}`
       : !this.body.grounded ? this.body.vy < 0 ? 'jump-02' : 'fall-00' : null;
@@ -62,6 +78,6 @@ export class Player {
     this.debug.clear();
     if (this.showDebug) this.debug.lineStyle(2, 0xffffff).strokeRect(this.body.x - BODY_WIDTH / 2,
       this.body.y - BODY_HEIGHT, BODY_WIDTH, BODY_HEIGHT);
-    this.label.setText(`vx ${this.body.vx.toFixed(1)} px/s | last ${this.lastSpeed.toFixed(1)} (${(this.lastSpeed / 43).toFixed(2)} m/s)\nheld ${this.motion.held.toFixed(2)}s | x ${this.body.x.toFixed(1)} y ${this.body.y.toFixed(1)}\njump ${this.air.height.toFixed(1)}px | ${this.body.sliding ? 'slide' : 'walk'} slope ${(Math.atan(this.body.slope ?? 0) * 180 / Math.PI).toFixed(1)}°`);
+    this.label.setText(`vx ${this.body.vx.toFixed(1)} px/s | last ${this.lastSpeed.toFixed(1)} (${(this.lastSpeed / 43).toFixed(2)} m/s)\nheld ${this.motion.held.toFixed(2)}s | x ${this.body.x.toFixed(1)} y ${this.body.y.toFixed(1)}\njump ${this.air.height.toFixed(1)}px | ${this.traversal.ladder ? 'climb' : this.body.sliding ? 'slide' : 'walk'} slope ${(Math.atan(this.body.slope ?? 0) * 180 / Math.PI).toFixed(1)}° | land ${this.air.hard ? 'hard' : 'normal'}`);
   }
 }
diff --git a/game/src/player/traversal.test.ts b/game/src/player/traversal.test.ts
new file mode 100644
index 0000000..1189eaa
--- /dev/null
+++ b/game/src/player/traversal.test.ts
@@ -0,0 +1,70 @@
+import { describe, expect, it } from 'vitest';
+import { stepBody, type Body } from './body';
+import { climb, dropThrough, traversal } from './traversal';
+import type { CollisionData, Geometry } from '../stage/collision';
+const dt = 1 / 120;
+const world: Geometry = { walk: [[[0, 500], [1000, 500]]], slide: [], oneWay: [[[100, 350], [300, 350]]],
+  climb: [{ x: 180, y: 353, w: 30, h: 147, kind: 'ladder' }], solid: [],
+  noGo: [], hazard: [], interact: [], spawn: [], ledge: [], glideLift: [] };
+const body = (y: number): Body => ({ x: 195, y, vx: 0, vy: 0, grounded: true });
+describe('vertical traversal', () => {
+  it('jumps upward through yellow lines and lands on top when falling', () => {
+    const b = { ...body(400), grounded: false, vy: -485 };
+    let crossed = false;
+    for (let i = 0; i < 120; i++) {
+      stepBody(b, [world], dt); if (b.y < 350) crossed = true;
+    }
+    expect(crossed).toBe(true); expect(b.y).toBe(350); expect(b.grounded).toBe(true);
+  });
+  it('drops through a yellow support without disabling the green floor', () => {
+    const b = body(350), state = traversal();
+    expect(dropThrough(b, state, [world])).toBe(true);
+    for (let i = 0; i < 120; i++) {
+      climb(b, state, [world], 0, 0, false, dt);
+      stepBody(b, [world], dt, state.drop > 0);
+    }
+    expect(b.y).toBe(500); expect(dropThrough(b, state, [world])).toBe(false);
+  });
+  it('handles Down slightly before Jump at a ladder top without grabbing it again', () => {
+    const b = body(350), state = traversal();
+    climb(b, state, [world], 1, 0, false, dt);
+    expect(state.ladder).not.toBeNull();
+    expect(dropThrough(b, state, [world])).toBe(true);
+    expect(climb(b, state, [world], 1, 0, true, dt)).toBe(false);
+    expect(state.ladder).toBeNull();
+  });
+  it('climbs slowly, holds position on release, and exits at the deck height', () => {
+    const b = body(500), state = traversal();
+    expect(climb(b, state, [world], -1, 0, false, dt)).toBe(true);
+    expect(b.y).toBeCloseTo(500 - 70 * dt); expect(state.ladder).not.toBeNull();
+    const held = b.y;
+    for (let i = 0; i < 30; i++) climb(b, state, [world], 0, 0, false, dt);
+    expect(b.y).toBe(held);
+    for (let i = 0; i < 300; i++) climb(b, state, [world], -1, 0, false, dt);
+    expect(b.y).toBe(350); expect(b.grounded).toBe(true); expect(state.ladder).toBeNull();
+    for (let i = 0; i < 300; i++) climb(b, state, [world], 1, 0, false, dt);
+    expect(b.y).toBe(500); expect(state.ladder).toBeNull();
+  });
+  it('releases a ladder on horizontal input or jump and ignores reserved ledges', () => {
+    for (const jumping of [false, true]) {
+      const b = body(400), state = traversal(); climb(b, state, [world], -1, 0, false, dt);
+      expect(climb(b, state, [world], 0, jumping ? 0 : 1, jumping, dt)).toBe(false);
+      expect(state.ladder).toBeNull(); expect(b.grounded).toBe(jumping);
+      expect(climb(b, state, [world], -1, 0, false, dt)).toBe(false);
+    }
+    const b = body(400), state = traversal();
+    climb(b, state, [{ ...world, climb: [], ledge: [{ x: 195, y: 400 }] }], -1, 0, false, dt);
+    expect(state.ladder).toBeNull();
+  });
+  it('traverses every authored Creek climb zone in both directions', () => {
+    const files = import.meta.glob<CollisionData>('../../../art/stages/rusty-creek/*.collision.json', { eager: true, import: 'default' });
+    for (const source of Object.values(files)) for (const box of source.climb) {
+      const b = { ...body(box.y + box.h), x: box.x + box.w / 2 }, state = traversal();
+      const geometry = { ...source, climb: [box] };
+      for (let i = 0; i < 1500; i++) climb(b, state, [geometry], -1, 0, false, dt);
+      expect(b.y).toBeGreaterThanOrEqual(box.y - 8); expect(b.y).toBeLessThanOrEqual(box.y + 8);
+      for (let i = 0; i < 1500; i++) climb(b, state, [geometry], 1, 0, false, dt);
+      expect(b.y).toBeGreaterThanOrEqual(box.y + box.h - 8); expect(b.y).toBeLessThanOrEqual(box.y + box.h + 8);
+    }
+  });
+});
diff --git a/game/src/player/traversal.ts b/game/src/player/traversal.ts
new file mode 100644
index 0000000..0ffde80
--- /dev/null
+++ b/game/src/player/traversal.ts
@@ -0,0 +1,55 @@
+import type { Box, Geometry } from '../stage/collision';
+import type { Body } from './body';
+export const CLIMB_SPEED = 70;
+export interface Traversal { ladder: Box | null; drop: number; detach: number }
+export const traversal = (): Traversal => ({ ladder: null, drop: 0, detach: 0 });
+
+function surfaceNear(geometry: Geometry[], x: number, y: number) {
+  const candidates: number[] = [];
+  for (const world of geometry) for (const line of [...world.walk, ...world.oneWay]) {
+    for (let i = 1; i < line.length; i++) {
+      const [a, b] = [line[i - 1], line[i]];
+      if (x < a[0] || x > b[0]) continue;
+      const height = a[1] + (x - a[0]) * (b[1] - a[1]) / (b[0] - a[0]);
+      if (Math.abs(height - y) <= 8) candidates.push(height);
+    }
+  }
+  return candidates.sort((a, b) => Math.abs(a - y) - Math.abs(b - y))[0] ?? y;
+}
+
+export function dropThrough(body: Body, state: Traversal, geometry: Geometry[]) {
+  if (!body.grounded && !state.ladder) return false;
+  const support = geometry.some(g => g.oneWay.some(line => line.slice(1).some((b, i) => {
+    const a = line[i];
+    return body.x >= a[0] && body.x <= b[0]
+      && Math.abs(body.y - (a[1] + (body.x - a[0]) * (b[1] - a[1]) / (b[0] - a[0]))) < (state.ladder ? 8 : 0.1);
+  })));
+  if (!support) return false;
+  state.ladder = null; state.drop = 0.22; body.y += 1; body.vy = 30; body.grounded = false;
+  return true;
+}
+
+/** Returns true while the ladder owns movement, including a stationary hold. */
+export function climb(body: Body, state: Traversal, geometry: Geometry[], vertical: number, horizontal: number,
+  jumpPressed: boolean, dt: number) {
+  state.drop = Math.max(0, state.drop - dt);
+  state.detach = Math.max(0, state.detach - dt);
+  if (state.ladder && (jumpPressed || Math.abs(horizontal) > 0.2)) {
+    state.ladder = null; state.detach = 0.2; body.grounded = jumpPressed; return false;
+  }
+  if (!state.ladder && vertical && Math.abs(horizontal) <= 0.2 && !jumpPressed && state.drop === 0 && state.detach === 0) {
+    state.ladder = geometry.flatMap(g => g.climb).find(b => body.x + 14 > b.x && body.x - 14 < b.x + b.w
+      && body.y >= b.y - 4 && body.y <= b.y + b.h + 8) ?? null;
+  }
+  if (!state.ladder) return false;
+  const ladder = state.ladder;
+  const x = ladder.x + ladder.w / 2;
+  const top = surfaceNear(geometry, x, ladder.y);
+  const bottom = surfaceNear(geometry, x, ladder.y + ladder.h);
+  body.x = x; body.vx = 0; body.vy = 0; body.sliding = false; body.slope = 0; body.grounded = false;
+  body.y = Math.max(top, Math.min(bottom, body.y + vertical * CLIMB_SPEED * dt));
+  if (vertical < 0 && body.y <= top || vertical > 0 && body.y >= bottom) {
+    state.ladder = null; body.grounded = true;
+  }
+  return true;
+}