As others noted, it only supported horizontal walls and floors. More specifically, though, it only supported _one_ gap (e.g., a single window or door). You couldn't stack floors on top of each other.

This allowed a single ray cast against the 2D map to learn which vertical slices of wall texture to paint (no pitch or roll, remember). Floors and ceilings could be painted in vertical order (above / below the horizon line - "floor" and "ceiling" being explicit for any given open area), iirc, and walls filled in based on the ray cast from the eye.

It can only draw horizontal floors and vertical walls (so the map is essentially 2D even though it is drawn in 3D). The player also cannot tilt their head sideways.

The engine is limited such that it fundamentally can't do up/down rotation of the camera.