The levels are stored as 6 bytes per "brush" (i.e. block): 1 byte per each x, y, z coordinates and width, depth and length[1]. The y (vertical) resolution of these values is a bit higher than the x/z resolution to allow for stairs while allowing for bigger levels on the horizontal plane. So the level files are not directly stored as a grid, but as a collection of axis aligned blocks.

The collision detection however builds a grid out of these, so the game can quickly look up any position within the game world[2].

[1] https://github.com/phoboslab/q1k3/blob/master/pack_map.c#L78... [2] https://github.com/phoboslab/q1k3/blob/master/source/map.js#...