What it is. The level is an actor. Spikes, fire, water, crumbling floors, gas — hazards that damage, block, or transform, and that obey rules the player can read, exploit, and turn against the enemies who share the space.

Player fantasy / why it’s fun. The room is a loaded gun. You learn where it points, then you turn it. The best hazard moment isn’t dying to a trap — it’s kiting a shopkeeper onto it. Mastery of the space outranks mastery of the fight.

When to use / when NOT

Use when:

• The space should carry difficulty so you can ship fewer, dumber enemies. A saw blade never needs an AI.
• You want pattern-emergence — hazards that touch enemies too produce stories you didn’t script.
• Procgen needs cheap, composable threat. Hazards tile; bespoke bosses don’t. See system-procgen-design.

NOT when:

• The game is about precise, legible duels (genre-fighting-game, genre-soulslike arenas) — a random gas cloud is noise, not tension.
• Hazards only punish the player and never the AI. That’s a tax, not a system — see Fails below.
• The threat must be authored beat-for-beat. Then it’s a system-telegraphs set-piece, not an environment.

Variants

Best when variants interact: a bomb (mechanic-throw) opens a floor, water pours in, the pour douses the fire an enemy was standing in. anchor-spelunky is the reference — every element is a verb that touches every other. anchor-minecraft and anchor-terraria make terrain the whole loop; anchor-nuclear-throne uses destructible cover as tactical texture.

Tuning levers

• Telegraph. How far ahead, and in what channel, the threat announces itself. Wind-up, floor crack, pressure hiss, red tint. Unfair without it; trivial with too much. Own the read via system-telegraphs and pattern-readability.
• Reversibility. Can the state undo? Fire burns out, ice thaws, floods drain, but a collapsed floor is forever. Reversible hazards invite play; permanent ones demand respect. Mix both.
• Enemy-affects-too. The single highest-leverage switch. If a hazard only threatens the player, it’s punishment; if it threatens all bodies, it’s a tool. Flip this on and encounters (system-encounter-design) design themselves.
• Trigger source. Contact, proximity, timer, weight, line-of-sight, or player action. Determines whether the hazard is dodged, waited out, or weaponized.
• Spread rate. For systemic hazards — how fast fire jumps tiles or water rises. Too fast reads as instant-death; too slow reads as scenery. Gate to reaction time.
• Density & placement. How much of the room is lethal. Procgen must guarantee a solvable path — see Verify. Pair scarcity with reward via pattern-risk-reward.
• Interaction matrix. Which hazards transform which (fire→ice→water→electricity). A small NxN table buys enormous system-emergent-systems depth. Keep it under 6 elements or it becomes antipattern-feature-soup.

Twist seams

• Hazards but they only hurt whoever moved last (constraint) — the floor is safe until you step; then the tile behind you arms. Turns a level into a genre-grid-puzzle of ordering. Pairs with mechanic-rewind to undo a bad step. See anchor-baba-is-you for rule-as-hazard framing.
• Environment but the player plants the hazards for enemies (perspective) — you don’t dodge traps, you set them. Bear traps, tripwires, lured pits. Now it’s genre-tower-defense wearing a genre-stealth coat; see anchor-dishonored for weaponized environment and mechanic-magnet for pulling foes into your kill zone.

How it wires to Hayao

• Model each hazard as a cell/entity in the scene tree carrying its own state (armed, burning, flooded, cracked). The visual — flicker, glow, spread animation — sets cosmetic = true so it stays out of world.hash(); the damage/spread logic stays in the deterministic sim.
• Systemic spread is a cellular tick: iterate hazard cells in a fixed, ordered pass and mutate next-frame state from current-frame neighbors. Never Math.random a spread — draw from the deterministic RNG so a seed replays identically.
• For grid-locked “whoever moved last” hazards, keep the rule in a pure Puzzle<State, Move> module so every layout is machine-proven survivable, mirroring examples/sokoban/.
• For continuous fire/fluid/collapse, study the physics and particle labs — read the matching sandboxes/<x>-lab for one mechanic in isolation before wiring several together.
• Destructible terrain must rebuild navigation after a change; enemy pathing (system-enemy-ai) reads the mutated grid, not the original.

Fails when…

• The hazard only hurts the player and never the AI → antipattern-boring-optimal (memorize the safe lane, ignore the system).
• No telegraph, or a telegraph in an unreadable channel → antipattern-input-lie and antipattern-rng-frustration; the death feels stolen.
• Systemic layers stacked past legibility (fire + gas + wind + collapse in one tile) → antipattern-feature-soup and antipattern-unreadable-juice.
• Procgen drops a hazard across the only path with no guaranteed solve → antipattern-difficulty-cliff; the run is dead on spawn.
• Permanent hazards with a checkpoint far behind → antipattern-fail-loop-tax. Pair reversibility with system-save-and-checkpoint and pattern-anti-frustration.
• The “trap” is just flat chip damage with no counterplay → antipattern-false-depth; it reads as depth, plays as a toll.

Verify

Prove it in docs/VERIFICATION.md. Assert on world.probe()/hash(): a seeded spread replays byte-identical; every procgen layout has a solver-proven survivable path; the enemy-affects-too switch actually deals damage to non-player bodies. The vision judge confirms telegraphs read from a headless SVG — see pattern-readability and system-telegraphs for the design bar those checks enforce.