Player movement

The player's body is driven by one motor: a piece of the simulation that, every tick, takes the buttons you are holding and works out where your body is now. It runs, jumps, crouches, climbs, swims, grips walls and swings on a rope, and it moves the way the classic shooters did: fast, with real control in the air.

4

movement modes: swim, climb, wall walk, on foot

60

times a second the motor moves the body

50°

the steepest slope the player can stand on

As of 2026-10-22. Enemies use a slightly stricter slope limit by default, about 46°.

On foot and in the air

On the ground, your body speeds up towards the direction you are pressing, laid along the slope you are standing on. In the air you can still steer, by the same rules Quake used, which is what makes strafe-jumping work: turning while you jump can carry you faster than running. Whether you are standing at all is decided by feeling for the floor under your feet, never by how fast you are moving, so walking up a ramp is not mistaken for a jump.

Your top speed on foot is worked out in one place, from a short list of things that can raise or lower it. The engine leaves several of them switched off, for a game to turn on: sprinting with stamina, extra jumps in mid-air, and jumping again the moment you land while the key is held.

None of these touch your steering in the air. That is deliberate: a speed bonus there would either break strafe-jumping or make it unfairly strong.

Stairs are climbed in one step, as part of the move. So that the view does not jolt upward on every stair, the camera eases up over the next moments while the body itself has already arrived.

Crouching, and one way to change size

A body changes size in exactly one place in the motor. Crouching, the smaller shape a swimmer takes, a status that shrinks or reshapes you, and a game swapping your body for another all go through it. Having one route matters because every size change must agree on the same questions: where your feet are, whether there is room overhead, and what a crouched and shrunken body is.

Growing is the careful direction. Before a body gets bigger, the motor checks that the larger shape would not end up inside a wall, a ceiling or the underside of a lift. If it would, the request waits and is tried again every tick, so letting go of crouch in a vent stands you up the moment you leave it. When you grip a wall, the wall is what holds still, so a change of size never pushes you into the surface you are clinging to.

Ladders, walls and water

A few rules settle the awkward cases. A ladder standing in a pool is still a pool: by default you swim, unless your body prefers the rungs. A body that cannot swim in a given liquid walks along the bottom instead, still taking its harm and still running out of breath. Near the surface you cannot float up out of the water; break the surface and you arc back down, as you would in a real pool. When you swim towards a ledge, the motor checks for something solid you could stand on, a floor, a door, a lift or a crate, and boosts you just enough to land on it. A body floating beside you is never treated as a ledge.

The ladders, liquids and gravity themselves are parts of the level, described on the level machinery sheet.

Lifts and the grapple rope

Riding a lift is not a mode. Each tick, before anything else, the lift's movement is added to your position, then you move as usual on top of it, so you can walk around a moving platform freely. Enemies, corpses, crates and loose items ride by the same rule. Jumping off a rising lift adds its rise to your jump; jumping off a falling one never robs you of your jump.

The grapple is a weapon that fires a hook (its firing side is on the weapons sheet). The motor owns the rope:

1. Hook in flightIf it has not caught anything within a short time, it is let go.
2. Hook catchesYour fall so far is forgiven, and the rope comes taut.
3. Reeling inThe rope pulls on you like a winch while you keep steering; close to the hook it goes slack and you hang.
4. Let goFire again, switch weapon, jump, die or go through a portal.

A hook that lands on a moving lift travels with it, and the rope follows. A game can let the hook catch a character too; if that character dies or goes through a portal, the rope is cut.

Falls

A landing is judged on the fastest you were falling in the last few moments, not over the whole drop, so a fall broken by a ledge halfway down is not charged as one long fall. Several things forgive a fall entirely: landing in deep water, grabbing a ladder, the grapple catching, going through a portal, gripping a wall, and respawning.

Fall damage is settled through the same single door as every other hurt in the game (see damage). The curve is shared too: players, enemies, corpses and props all use the same arithmetic.

The player and the enemies

The engine has a standing rule that the player's body must be a body like any enemy's, with no mechanism that one can reach and the other cannot. Their shapes, their stances, how they change size, how they ride lifts, how they decide to swim and how a fall hurts them are the same machinery. What differs is who is steering: you move by the classic shooter rules described above, while an enemy moves by its own arithmetic, guided by its decisions and its route through the level.

• Bodies and collisionThe shapes the motor moves, and how it slides along walls.
• Level machineryThe lifts, ladders, liquids and portals the body moves through.
• What enemies can doHow enemies chase, climb, swim and swarm.