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 for the spawn or for any enemy already walking; recompute paths when towers change (refs §1).
• Fast enemies slow down at corners or skip waypoints → per-step movement stops at each waypoint or overshoots it. Carry the leftover distance to the next waypoint (refs §1).
• 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-navmesh for nav-based pathing.
• Map: godot-tilemap / unity-tilemap-2d for the grid/lanes; level-design for map layouts.
• Data: godot-resources / unity-scriptableobjects to define towers, enemies, and waves as assets.
• UI: game-ui-ux for HUD layout, scaling, and the build menu; godot-ui-control for the concrete widgets (lives, gold, wave preview).
• Polish: audio-design for fire/hit/leak sound cues.

References

• For path representations (waypoints / A* / flow fields), targeting modes, DPS and economy math, enemy scaling, and wave pacing, read references/balancing.md.