bevy-genre-tower-defense

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.