Bevy 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 Update or 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 on latest; 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 latest branch.
• ECS - Component, systems, Query, Startup, Update, and .chain().
• Plugins - Plugin, DefaultPlugins, MinimalPlugins, and add_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.