Level machinery

A level has lifts that carry you, doors that lock, crates that burst, buttons you press or shoot, ladders, pools of water and lava, rooms with their own gravity, and portals. The engine ships all of these as working machinery; a level only places them and sets their numbers.

Everything on this sheet is part of the simulation, so it ticks in step with everything else and replays exactly. Lifts move first in every tick, so anything standing on one is carried by a movement that has already happened.

Lifts, crushers and staircases

A mover is a solid box that travels along a list of stops. The same part makes a passenger lift, a crusher, a ram that slides across a room, and the slab a door is built on. A level chooses how it travels: stop at every stop and wait for a button, keep going round for ever, or "go, come back and stop" for a one-press crusher. A row of movers can also be launched one after another to make a rising staircase.

Most things ride a mover's top: the player, enemies, corpses, crates, dropped items and even loose chunks of gore. When a mover meets something in its way, what happens depends on what that thing is. A body or crate with room to move is pushed along, and dropped items and chunks of gore are shoved aside without harm. Something with health and no room left is crushed, in the way the level chose.

A level can also pause a mover where it stands, and any button press that sends it somewhere releases the pause. Lift buttons can call a lift, send it away, or pause it, and several slabs wired to one button move in step.

Doors, props and controls

A door is a set of rules on top of a mover's slab: where its closed and open positions are, whether it is locked, and which key opens it. A key can be good for one door, several, or any number, and a door can relock itself every time it closes. A door can open itself when a suitable enemy comes near, and close again after a delay, trying again if somebody is standing in the way. The player never opens a door by walking at it; you press use. A door can be given health so that shooting it opens it or breaks it, but a locked door is never opened by gunfire, only broken. A level can leave a gap under a door for small things to pass.

Props are the barrels and crates of a level. They have health, can be pushed and ride lifts, and when destroyed they drop their loot and, if they are explosive, blow up, credited to whoever destroyed them. A chain of exploding barrels always plays out the same way. A prop can come back after a while, but not on top of somebody: it waits until the spot is clear.

Enemies step on plates too, not only the player. A control can have any mix of these, or none at all, which makes it pure decoration. Whether pressing it does anything is decided inside the tick, never by the picture on screen.

Ladders, liquids and gravity

Most of what remains is built from zones: invisible boxes that change how the simulation treats whatever is inside them.

• Ladders. A ladder is a thin zone laid against a wall; touching it is climbing it. A rope is a ladder you can climb from every side. A ladder can ride a lift, carrying its climber with it, and a ladder can be built in sections that are shot away, and mended, one section at a time. An enemy climbs a ladder only when its route goes up it.
• Liquids. A game registers its own liquids; the engine offers ready-made water, lava and poison. Each liquid states how it swims, whether it drowns you, whether and how often it hurts, and how thick it is: only a thick liquid slows a body wading through it. Corpses and chunks can float on a liquid dense enough to carry them.
• Spikes. A zone that hurts each time a body comes into contact with it.
• Gravity. A room can pull harder or more lightly than the rest of the level. It multiplies everything at once, so in a room at a fifth of normal gravity, bodies, rockets, chunks and crates all weigh a fifth as much and keep their differences. A room at zero is weightless.

How a body actually climbs, swims and wades is on the player movement sheet.

Portals

A portal is a pair of boxes linked to each other. It takes anything with a position, not only players, and keeps its momentum, turned to face out of the other end.

After a trip, both ends refuse that one traveller for a moment, so you are not thrown straight back. A body that arrives on top of somebody kills them: the telefrag, settled through the one damage door like any other hurt. Friendly enemies queue politely at a portal so as not to telefrag each other; hostile ones do not.

The area pause and level orders

Two tools let a level steer its enemies without changing how they think.

The area pause puts chosen enemies, and their corpses, to sleep: those of a certain kind, or in a certain part of the level, or both. It is a rule the level or game sets and lifts, re-checked every tick, so an enemy carried out of a paused region by a lift wakes up on its own. The player is never paused.

Level orders let a zone hand a short script to an enemy that walks into it: hold here, wait for a lift or a door or a number of seconds, walk to a point, fire at a point, then carry on as normal. A condition can go with the script, such as "only while it has not noticed you", so an enemy that gets shot drops the script and fights. Orders never change what an enemy is able to do; they only take the decision for a while. How enemies decide for themselves is on the how enemies decide sheet.

• Player movementRiding, climbing, swimming and wading, from the body's side.
• NavigationHow enemies find routes through doors, up ladders and across portals.
• Bodies and collisionWhat the boxes and bodies in a level are made of.