Portal / Linked Doors — Design Codex

What it is. Two openings are welded into one seam: whatever crosses opening A exits opening B at the same speed, rotated to B’s facing. Space folds; the level’s true topology is not its layout.

Player fantasy / why it’s fun. You see through a wall to somewhere else, aim at it, and arrive — the “speedy thing goes in, speedy thing comes out” loop of Portal. The joy is turning a dead-end room into a launch tube by re-parenting gravity through geometry.

The verb

Aim a portal at a surface, then walk, fall, or fling yourself through the resulting seam.

How it feels / why it’s fun

Momentum is preserved — a long drop into a floor portal fires you out a wall portal at that same speed. Flinging beats walking; that gap is the whole skill ceiling.
The sightline is the tutorial. If the player can see the far room framed inside the near portal, they already understand the connection. Break that view and you break comprehension — see pattern-readability.
One rule, deep tree. A single “in-here-is-out-there” law composes into puzzles nobody hand-authored. That’s pattern-emergence doing the design work.

Tuning levers

Lever	What it changes	Sane default
Momentum carry	fraction of speed kept through the seam	1.0 (lossless — the fun)
Placeable count	portals the player owns at once	2 (one pair)
Placement surfaces	which walls accept a portal	”portalable” material only
Exit-facing rule	how velocity rotates on exit	align to exit normal
Cooldown / recharge	delay before re-firing	0; instant is snappier
Who may pass	player / objects / enemies / light	player + objects
Persistence	does a portal survive room reset	pair persists until re-fired

Slots into

genre-puzzle-platformer — the native home; folded rooms plus mechanic-double-jump-free traversal.
genre-grid-puzzle — quantized portals as discrete edges of a graph; pairs with system-map-and-navigation.
genre-metroidvania — a portal gun is a gated traversal power; place it behind system-progression.
genre-immersive-sim — portals as one of several ways past a wall; rewards pattern-emergence.
Anchor: anchor-portal is the canon. Study its “momentum in, momentum out” and its blue/orange colour-coding.

Twist seams

Portals but only enemies can pass through, not you (constraint) — the seam is a hazard, not a tool: you redirect their patrols and projectiles, never step through. Kin to a deflect (mechanic-deflect) at level scale; leans on system-enemy-ai and genre-stealth.
Portal but placing one closes the last (structure) — you never hold a pair; you hold a single moving door and must chain placements, so the puzzle is sequencing placements in time, not arranging them in space. Turns the mechanic into a mechanic-teleport with a breadcrumb tax.
Portals but light and sightlines pass, mass does not (constraint) — you route lasers, guard vision cones, and camera views through the fold while your body stays put; pairs with system-telegraphs and stealth detection.

How it wires to Hayao

Model the pair as linked transforms: on cross, remap position into the partner’s local frame and rotate velocity by the frame delta. Keep the math in pure logic so genre-grid-puzzle variants stay machine-provable.
For the discrete case, treat portals as extra edges in the level graph and prove reachability with the same solver you’d use for any turn-based puzzle — the sokoban example is the reference for the logic/view split.
The seam preview (rendering the far room inside the near opening) is pure view — mark those nodes cosmetic so they stay out of the world hash. Study a camera/render lab in sandboxes/ for the framing trick rather than a whole game.
Momentum-through-a-portal is a physics coupling; prototype the carry rule in a physics sandboxes/ lab before wiring it to a level.
Determinism note: portal traversal must resolve identically every run — order the “who crossed this tick” checks; no wall-clock, all randomness through the world RNG.

Fails when…

Momentum is dropped on exit. Lossless carry is the mechanic; bleed it and you’ve built a fancy mechanic-teleport. Don’t silently cap speed — that’s an antipattern-input-lie.
The far side can’t be read. No sightline, no preview, or a disorienting exit-facing means the player fires blind and rages. See pattern-readability and pattern-fairness-and-trust.
Every surface is portalable. Infinite placement collapses the puzzle; the constraint (only some walls accept portals) is what creates the problem. Unbounded freedom here is antipattern-false-depth.
Too many pairs at once. Three-plus open seams overwhelm spatial reasoning and invite antipattern-decision-paralysis; the two-portal limit is load-bearing.
The twist isn’t the whole game. A portal bolted onto an unrelated loop is antipattern-feature-soup — fold the geometry into the core verb or cut it. See process-the-twist.