Bevy Genre Tower Defense Agent Skill
Bevy genre router for tower defense. Use when placing towers, marching creeps along waypoints, and spending gold on a fixed timestep. Keywords: Bevy, tower defense, creep, waypoint, gold, ECS.
Install
one command, takes just this skill from the repositorynpx skills add https://github.com/thedivergentai/Game-Design-Agentic-Skills --skill bevy-genre-tower-defenseWhat comes with it
538 bytes besides the instructionThe instruction itself
18 sections, as written by the authorBevy Tower defense (thin router)
Genre router. Domain work lives in the Bevy skills below.
When to use
Load this for towers, creeps, and waves on a path.
ECS shape
- Components hold the genre's per-entity flags as plain data. This file does not name new engine types.
- Resources hold run-wide counters (score, wave index, seed) via bevy-data-resources.
- Systems read those components on
Updateor the fixed timestep from bevy-master. Order them with.chain()when a later system must see the result. - Schedule: gameplay mode changes go through bevy-state-machines. Do not invent a genre plugin.
Coming from Godot
| Godot idea | Bevy idiom |
| --- | --- |
| Genre scene tree | Entities plus the domain skills linked below |
| Autoload director | A resource, not a singleton node |
| Signal bus for the loop | bevy-events-messaging |
Routing
- bevy-game-loop-waves - Creep waves.
- bevy-navigation - Paths and navigation meshes.
- bevy-combat-system - Tower shots.
- bevy-economy-system - Build costs.
Scope
Towers, creeps, waypoints, and gold are plain components and one resource. A navigation mesh belongs on bevy-navigation.
Examples
[package]
name = "tower-defense"
version = "0.1.0"
edition = "2021"
[dependencies]
bevy = "0.20"
Creeps walk a waypoint list. Towers that are off cooldown and in range spend a shot and damage the nearest creep. Gold is one resource.
#[derive(Component)]
struct Tower {
range: f32,
cooldown: f32,
damage: i32,
}
#[derive(Component)]
struct Creep {
hp: i32,
waypoint: usize,
speed: f32,
}
#[derive(Resource)]
struct Path {
points: Vec<Vec3>,
}
#[derive(Resource)]
struct Gold(i32);
fn march_creeps(
time: Res<Time<Fixed>>,
path: Res<Path>,
mut creeps: Query<(Entity, &mut Transform, &mut Creep)>,
mut commands: Commands,
) {
let dt = time.delta_secs();
for (entity, mut transform, mut creep) in &mut creeps {
let Some(goal) = path.points.get(creep.waypoint) else {
commands.entity(entity).despawn();
continue;
};
let delta = *goal - transform.translation;
let step = creep.speed * dt;
if delta.length() <= step {
transform.translation = *goal;
creep.waypoint += 1;
} else {
transform.translation += delta.normalize() * step;
}
}
}
fn towers_fire(
time: Res<Time<Fixed>>,
mut towers: Query<(&Transform, &mut Tower)>,
mut creeps: Query<(&Transform, &mut Creep)>,
) {
let dt = time.delta_secs();
for (tower_tf, mut tower) in &mut towers {
tower.cooldown = (tower.cooldown - dt).max(0.0);
if tower.cooldown > 0.0 {
continue;
}
let mut best: Option<(f32, Mut<Creep>)> = None;
for (creep_tf, creep) in &mut creeps {
let dist = tower_tf.translation.distance(creep_tf.translation);
if dist > tower.range {
continue;
}
let closer = match &best {
Some((best_dist, _)) => dist < *best_dist,
None => true,
};
if closer {
best = Some((dist, creep));
}
}
if let Some((_, mut creep)) = best {
creep.hp -= tower.damage;
tower.cooldown = 0.6;
}
}
}
Startup inserts the path and gold, then spawns one tower and one creep. The two systems share Creep, so they run in one chain.
#[derive(Component)]
struct Tower {
range: f32,
cooldown: f32,
damage: i32,
}
#[derive(Component)]
struct Creep {
hp: i32,
waypoint: usize,
speed: f32,
}
#[derive(Resource)]
struct Path {
points: Vec<Vec3>,
}
#[derive(Resource)]
struct Gold(i32);
fn setup(mut commands: Commands) {
commands.insert_resource(Path {
points: vec![
Vec3::new(0.0, 0.0, 0.0),
Vec3::new(8.0, 0.0, 0.0),
Vec3::new(8.0, 0.0, 8.0),
],
});
commands.insert_resource(Gold(100));
commands.spawn((
Tower {
range: 5.0,
cooldown: 0.0,
damage: 10,
},
Transform::from_xyz(4.0, 0.0, 2.0),
));
commands.spawn((
Creep {
hp: 30,
waypoint: 0,
speed: 2.5,
},
Transform::from_xyz(0.0, 0.0, 0.0),
));
}
fn march_creeps(
time: Res<Time<Fixed>>,
path: Res<Path>,
mut creeps: Query<(Entity, &mut Transform, &mut Creep)>,
mut commands: Commands,
) {
let dt = time.delta_secs();
for (entity, mut transform, mut creep) in &mut creeps {
let Some(goal) = path.points.get(creep.waypoint) else {
commands.entity(entity).despawn();
continue;
};
let delta = *goal - transform.translation;
let step = creep.speed * dt;
if delta.length() <= step {
transform.translation = *goal;
creep.waypoint += 1;
} else {
transform.translation += delta.normalize() * step;
}
}
}
fn towers_fire(
time: Res<Time<Fixed>>,
mut towers: Query<(&Transform, &mut Tower)>,
mut creeps: Query<(Entity, &Transform, &mut Creep)>,
) {
let dt = time.delta_secs();
for (tower_tf, mut tower) in &mut towers {
tower.cooldown = (tower.cooldown - dt).max(0.0);
if tower.cooldown > 0.0 {
continue;
}
let mut best_dist = f32::MAX;
let mut best_entity = None;
for (entity, creep_tf, _) in creeps.iter() {
let dist = tower_tf.translation.distance(creep_tf.translation);
if dist <= tower.range && dist < best_dist {
best_dist = dist;
best_entity = Some(entity);
}
}
let Some(entity) = best_entity else {
continue;
};
let Ok((_, _, mut creep)) = creeps.get_mut(entity) else {
continue;
};
creep.hp -= tower.damage;
tower.cooldown = 0.6;
}
}
pub struct TowerDefensePlugin;
impl Plugin for TowerDefensePlugin {
fn build(&self, app: &mut App) {
app.add_systems(Startup, setup)
.add_systems(FixedUpdate, (march_creeps, towers_fire).chain());
}
}
Selling a tower pays gold and despawns it. One system owns both writes.
#[derive(Component)]
struct Tower {
range: f32,
cooldown: f32,
damage: i32,
}
#[derive(Resource)]
struct Gold(i32);
fn sell(
keys: Res<ButtonInput<KeyCode>>,
mut gold: ResMut<Gold>,
towers: Query<Entity, With<Tower>>,
mut commands: Commands,
) {
if !keys.just_pressed(KeyCode::KeyX) {
return;
}
let Some(entity) = towers.iter().next() else {
return;
};
gold.0 += 40;
commands.entity(entity).despawn();
}
Expert Knowledge Delta
Target: Bevy 0.20+.
Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.
Who owns state
| Piece | Owner | Who may mutate | Schedule |
| --- | --- | --- | --- |
| Data on one entity | Component | One system via Query | Update or FixedMain |
| One value for the whole app | Resource | One system | Startup to insert, then the schedule that reads it |
| Spawn | Commands | The spawning system | Startup for the initial world; later schedules for runtime spawn |
| Order that must not race | .chain() | The chained systems only | Same schedule |
| Plugin boundary | Plugin | build registers systems and resources | Before the app runs |
Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.
Decision
| If you need | Do this | Do not |
| --- | --- | --- |
| Per-entity data | Component + Query | A Godot node script |
| App-wide config or clock | Resource | Copy the value onto every entity |
| Stable simulation step | FixedMain and the Fixed clock | Tie simulation to raw frame Update |
| Startup spawn | Commands in Startup | Spawn from a render system |
| Cross-system notice that is not shared state | Message or observer slot in bevy-events-messaging | A new event type invented in this file |
| An API outside the documented set | Check the official docs first | A made-up crate or API |
Load references/ecs-ownership.md for the ownership table.
NEVER
- Do not pin
/0.20.0/in docs URLs. WHY: links stay onlatest; the 0.20 target is stated once in prose. - Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless
.chain()orders them. - Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.
- No Godot types in Bevy code. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
- No invented genre APIs. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
- Do not pick a physics or net crate here. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
Reference
Official Documentation
bevy crate- API reference and Cargo feature list.- Assets - Official asset catalog. No third-party crate was selected.
- Examples - Example tree on the
latestbranch. - ECS -
Component, systems,Query,Startup,Update, and.chain(). - Plugins -
Plugin,DefaultPlugins,MinimalPlugins, andadd_plugins.
Related Skills
Prerequisites
- bevy-master - ECS ownership and schedules.
- bevy-ecs-composition - Spawn and query shape.
Complements
- godot-genre-tower-defense - Equivalent Godot skill.
- bevy-game-loop-waves - Creep waves.
- bevy-navigation - Paths and navigation meshes.
- bevy-combat-system - Tower shots.
- bevy-economy-system - Build costs.
Downstream / consumers
- None. This router does not own a second implementation.
Master
- bevy-master - Bevy router.
How to use it
Take thedivergentai/bevy-genre-tower-defense from the repository into ~/.claude/skills for personal
use, or into .claude/skills inside a project.
The agent identifies a skill by the name field in its header. Two skills with the
same name cannot sit side by side — one of them will be ignored.