/tower-defense

name: tower-defense
description: >
  Build a tower defense: enemies pathing along lanes, wave spawning, towers that auto-target and fire,
  an economy, and lives. Use for a tower-defense/wave-defense game, or balancing waves and economy.

Tower Defense

A playbook for tower defense — enemy pathing, wave spawning, tower targeting, and the economy that ties them together. This is a **compositional** skill: it orchestrates pathing, AI, and UI into a TD loop. It does not re-teach pathfinding; it defines the systems and the balance levers (DPS vs. HP vs. income) that decide whether a TD is tense or trivial.

When to use

• Use when building a game where the player **places towers** that automatically attack

**waves of enemies** following a path, spends earned currency to expand/upgrade, and loses if too many enemies leak through.

• Use when designing wave composition/pacing, tower targeting priorities, or the gold economy.

**When _not_ to use:** the player directly controls a shooter → `fps-shooter`. Free-form base building with needs → `survival-crafting`. For the pathfinding algorithm itself, use `game-ai`.

Core loop

**Prepare (place/upgrade towers with current gold) → start the wave → enemies path toward the goal while towers auto-fire → earn gold from kills → survive the wave → repeat against a harder one.** The tension is a planning puzzle: is my current DPS enough for what's coming, and can I afford the answer?

Must-have systems

1. **Enemy path** — waypoint lanes, or grid + pathfinding for maze TD (refs). 2. **Wave spawner** — timed, scripted sequences of enemy types with per-wave scaling. 3. **Towers** — range, fire rate, damage, projectile/hitscan, splash; placement validity. 4. **Targeting** — per-tower priority (first/last/closest/strongest/weakest) (refs). 5. **Economy** — gold from kills + wave bonuses; tower/upgrade costs; sell refund. 6. **Lives / leak** — enemies reaching the goal cost lives; 0 = game over. 7. **Wave/prep UI** — lives, gold, next-wave preview, start-next-wave control.

Design knobs

| Knob | Effect | Notes | | ------------------------- | ----------------------- | ---------------------------------------- | | Enemy HP growth/wave | upgrade pressure | Geometric ~1.1–1.2× (refs). | | Income vs. HP curves | difficulty | Player should _almost_ afford each wave. | | Tower DPS / range / cost | tower identity | Cheap+wide vs. expensive+tall. | | Targeting priority | optimal placement | Changes play more than raw stats. | | Enemy variety | counters one-build wins | Fast / armored / flying / swarm / boss. | | Leak penalty | stakes | Cost lives _and_ lost bounty. | | Upgrade vs. build economy | strategy depth | Diminishing upgrade returns. | | Wave pacing | tension curve | Spike → breather → spike, not monotonic. |

Patterns

1. Wave spawner (timed, data-driven)

# Pseudocode. A wave is data: a list of (enemy_type, count, spacing). Spawn over time.
wave = [ ("grunt", 10, 0.5), ("runner", 5, 0.3), ("tank", 2, 1.0) ]

def run_wave(wave):
    for enemy_type, count, spacing in wave:
        for _ in range(count):
            spawn_enemy(enemy_type, at=path[0])   # enter at the path start
            wait(spacing)                          # seconds between spawns (use a timer/coroutine)
    # wave clears when all spawned enemies are dead or have leaked

2. Tower targeting (filter to range, then pick by priority)

# Pseudocode. "First" (furthest along path) is the standard default for stopping leaks.
def acquire_target(tower, enemies, mode="first"):
    in_range = [e for e in enemies if distance(tower.pos, e.pos) <= tower.range]
    if not in_range: return None
    if mode == "first":    return max(in_range, key=lambda e: e.progress)   # furthest along path (stop leaks)
    if mode == "last":     return min(in_range, key=lambda e: e.progress)   # least progress (guard the entrance)
    if mode == "closest":  return min(in_range, key=lambda e: distance(tower.pos, e.pos))
    if mode == "strongest":return max(in_range, key=lambda e: e.hp)         # burn down tanks first
    if mode == "weakest":  return min(in_range, key=lambda e: e.hp)         # secure kills / last-hit bounty
    return in_range[0]

3. "Can this lane hold?" sanity check (DPS vs. HP)

# Pseudocode. One tower's damage to one enemy crossing its range.
tower_dps     = tower.damage * tower.fire_rate
time_in_range = tower.range_coverage_length / enemy.speed
damage_dealt  = tower_dps * time_in_range
# Lane holds if summed damage_dealt from covering towers >= enemy.hp (per enemy). See refs.

Pitfalls / failure modes

• **Enemy HP scales slower than player income/DPS** → game trivializes after a few waves.

Tune the HP curve against the income curve (refs).

• **One dominant tower/strategy** → no decisions. Add enemy types that hard-counter builds

(armored vs. rapid-fire, flying vs. ground-only) and diminishing upgrade returns.

• **Maze TD that can fully block the goal** → enemies get stuck/softlock. Forbid placements

that sever the path; recompute paths when towers change (refs).

• **No next-wave telegraph** → specials feel random and unfair. Preview upcoming composition.
• **Per-frame movement unscaled by `dt`** → speed varies with frame rate. Scale steering by `dt`.
• **Leaks only cost lives, not gold** → leaking is sometimes optimal. Make leaks lose bounty too.
• **Monotonic difficulty ramp** → exhausting. Pace spikes and breathers.

Composition (build it from these skills)

• **Pathing:** `game-ai` for A\*/flow-field/steering; for fixed lanes, simple waypoint following.
• **Enemy movement:** `godot-2d-movement` (or engine equivalent) to move along the path; `unity-navmes