Tower Defense

What it is. Enemies walk a path; you spend a shared economy building towers that fire on them. The whole game is where you place what — a spatial optimisation puzzle that plays out in real time across escalating waves.

Player fantasy / why it’s fun. “I read the wave, I built the counter, I watched my machine hold the line.” The satisfaction is a plan surviving contact — then the finale wave that almost breaks it and doesn’t.

Pillars

Counters must bite. A counter is near-hard, not a soft resist. Soft resists get erased by tower-count scaling — if stacking one tower beats everything, there are no build decisions.
Coverage is geometry. range × distance-to-lane = the fire-window chord. A tower’s worth is how long it can shoot each enemy. The range ring is the genre’s most important UI.
Waves breathe. Pressure then relief. Runner waves are breaks; the curve ramps in envelope, not monotone HP. A wall of ever-bigger numbers is a spreadsheet, not tension.

The loop stack

Scale	The beat
Moment	Place/upgrade a tower; watch its ring cover the lane; a leaker slips and you patch the gap.
Encounter	A wave: read its archetype mix → the economy you saved buys the counter → it holds or leaks.
Session	A map, ~10 waves: opening eco vs defence tension, a midwave spike, a finale that peaks.
Meta	Unlock towers, maps, difficulty tiers; carry a build-order intuition between runs.

Essential systems

System	Why it’s load-bearing
`system-counter-systems`	The duel matrix. Each enemy archetype has a near-hard answer; picking it is the decision.
`system-enemy-archetypes`	The alphabet a wave is spelled from — fast/armored/flying/swarm/shielded.
`system-difficulty-and-dda`	The wave curve: ramp, breathers, finale peak — gate on shape, not raw HP.
`system-economy`	The spend that makes placement a choice — build now vs save for the counter.
`system-encounter-design`	Composing archetypes into a wave that demands a mixed build.

Content & difficulty model

Wave = an archetype recipe. Author waves as mixes (8 fast + 2 armored), not HP totals. Difficulty is the mix forcing a broader build, plus tighter timing.
Gate the curve on shape. Assert “each wave ≥ 55% of the previous, finale peaks” — not monotone HP. Runner waves are intentional dips.
The bare-lane baseline. A lane with no towers must fall early (null-strategy proof, FUN.md law 4). A “threat” a do-nothing survives isn’t a threat.
Mixed beats mono. The intended mixed build survives 10/10; a mono build on a bigger budget fails. That gap is the proof the counters matter.

Signature-mechanic seeds

TD but you also walk the path (perspective) — a hero unit you steer between builds; placement and positioning share your attention.
TD but the towers are the enemies’ path (mechanic-swap) — every tower you place reshapes the maze; maze-building becomes the core decision.
TD but the wave is a deck you drafted (structure) — you build the attack for a rival lane and defend theirs. Pairs system-build-diversity.
TD but ammo is a shared resource that runs dry (constraint) — towers compete for a finite magazine; over-building starves the finale.
TD but towers age and crack (theme + tonal) — weathered stonework that ages each wave; repair vs expand becomes the economy tension.

The strongest TD twist bends the decision, not the theme: change what the player builds against (a rival’s drafted wave), what the towers cost (a shared magazine), or where the path goes (maze-building). See process-the-twist.

Common pitfalls

Soft counters. A 20% resist vanishes under stacking. Make the counter near-hard or the build collapses to “spam the best tower.”
Monotone HP ramp. Waves that only grow read as a slog. Breathers and a peaked finale are the pacing.
Invisible range. Without a visible ring, placement is guesswork and coverage-as-geometry is illegible.
Economy with no tension. If you can always afford everything, placement isn’t a decision — starve the player enough to force priorities.
Global upgrades over placement. If flat “+damage” buttons beat clever positioning, coverage-as-geometry is dead. Keep the map the puzzle.

Anchors

anchor-into-the-breach — perfect-information spatial threat-reading; TD is the real-time cousin. Steal its “read the telegraph, answer with placement.”
system-counter-systems‘s duel matrix is the balance skeleton — treat each tower↔archetype pair as a proven duel.

Verify

Prove it in FUN.md §8 · Tower defense: mixed build survives 10/10, a bigger-budget mono build fails, the bare lane falls early, and counter duels resolve from both sides.

Composes with

system-counter-systems — the near-hard duel matrix the whole game rests on.
system-enemy-archetypes — the archetype alphabet waves are written in.
system-difficulty-and-dda — the breathing wave curve.