Twin-stick Horde Survival (Vampire Survivors-like)

What it is. You move; you don’t aim. Weapons fire automatically. The screen fills with enemies at a rising rate, and every few seconds you pick one upgrade from a draft. The run is a race between the swarm’s growth and your build’s growth — survive the timer, or drown.

Player fantasy / why it’s fun. “My build came online.” The turn from scraping by to mowing hundreds — the multiplicative moment when three upgrades click and the horde that was killing you becomes XP confetti. Minimal input, maximal escalation.

Pillars

Positioning is the only input. No aiming, no reloading — the skill is orbiting the horde so your auto-attacks sweep them, herding density into your fire. (Kiting bots corner themselves; the skill is the orbit — FUN.md §6.)
Build growth is multiplicative. Upgrades compound — +area × +count × +damage — so the power fantasy accelerates. This is why the tide must be superlinear to keep pace (system-build-diversity).
The tide always rises. Spawn pressure climbs relentlessly (quadratic ramp) so the run is always tightening; the peak-alive count is a designed target, not an accident.

The loop stack

Scale	The loop
Moment	Orbit; sweep a cluster; scoop the XP gems it drops.
Encounter	A wave / minute band: survive its density, then the level-up draft that answers it.
Session	One ~10–20 min run: draft a build, out-scale the tide, reach the timer or die.
Meta	Between-run persistent unlocks (new characters, weapons, permanent perks) that change what you can draft.

Essential systems

System	Why it’s needed
`system-build-diversity`	The draft is the game; many upgrades × real synergies make each run a different build story. Pillar 2.
`system-meta-progression`	Persistent between-run unlocks that add options (not just power) — the reason to run again. Pillar of retention.
`system-reward-schedules`	XP gems, chests, level-up timing — the drip that paces the power fantasy; keep it ethical (no dark patterns).
`system-difficulty-and-dda`	The tide curve; the quadratic spawn ramp is the difficulty. Assist = slower ramp or a revive.
`system-procgen-design`	Enemy composition and pickup placement varied per run within provable pressure bounds.
`system-session-structure`	A run is a fixed-length timer; define the band structure (minute → boss-minute → finale).

Content & difficulty model

Spawn pressure must be superlinear. Multiplicative build growth out-runs a linear tide by minute three; the horde needs a quadratic ramp to stay a threat through the finale (FUN.md §6). Derive the ramp from expected build DPS, not by feel.
Level-up picks pause the sim, and are input actions. The draft freezes time; each pick is a replayable input, never a direct mutation (FUN.md §6, law 6).
Assert peak alive ≥ N. Lock in the horde feel so it can’t silently regress — a build that trivialises the swarm should still face a wall of bodies at the peak (FUN.md §6).
Synergies over stat sticks. A weapon that combines with another (garlic + knockback = a grinder) beats a flat +10% damage. Build the upgrade pool so 3–4 archetypes each have a payoff spike.

Reference wiring: examples/emberwake — the horde sim in world.state with a pooled-sprite view (the cosmetic-view rule under load); the orbit-bot survival proof and peak alive assertion pattern. Grep docs/API.md for the pooling and particle primitives.

Signature-mechanic seeds

“X but Y” (process-the-twist) — bend the input, the tide, or the draft.

Survivors but you draw your attack path — one gesture per few seconds aims the sweep; positioning plus one deliberate stroke. (mechanic-swap)
Survivors but the horde is your XP economy AND your clock — kills you don’t scoop become the enemies of the next minute. (structure)
Survivors but you draft the enemies too — pick your upgrade and what the tide sends next; risk-reward push-your-luck. (mechanic-swap — pairs with pattern-risk-reward)
Survivors but two players share one XP pool and one screen — coop where you fight over the draft. (perspective — pairs with genre-coop-chaos)
Survivors but the map shrinks each minute — the arena, not just the spawn rate, is the rising tide. (constraint)

Common pitfalls

Linear tide, exponential build. By minute three the player is a god and bored. The ramp must be superlinear (FUN.md §6). This is the #1 failure.
Aiming. Adding a manual aim verb breaks the minimal-input fantasy and the orbit skill. Keep the hands quiet; the mind does the building.
Flat upgrade pool. All-stat-sticks, no synergies → every run is the same build. Author payoff spikes (system-build-diversity).
Meta that only adds raw power. If unlocks just make you stronger, runs homogenise. Add options that change the draft (system-meta-progression).
A draft that doesn’t pause. Picking under fire is stress, not strategy — freeze the sim for the choice.

Anchors

anchor-vampire-survivors — the minimal-input horde loop; build-growth vs rising tide; the auto-attack fantasy.
anchor-nuclear-throne — arcade twin-stick roguelite tightness; steal the run-based mastery and moment-to-moment feel.

Verify

Prove it with FUN.md §6 · Horde survival — orbit-bot survives with an hp floor; peak alive ≥ N asserted so the horde can’t regress; the superlinear spawn ramp checked against build DPS; sim ms/step budget under load. Design the tide-vs-build here; prove the race there.

Composes with

system-build-diversity — the draft is the game; synergies over stat sticks.
system-meta-progression — between-run options are the retention loop.
pattern-feedback-loops — the build’s positive loop must stay just behind the tide.