Since Doom was not a fully 3D engine, it was considered to be a 2.5D game. This 2.5D engine design limited Doom to relatively simple non-overlapping map designs since it is not possible for there to be an exposed bridge or balcony that the player can both see over and walk under. In Doom, only a simple moving platform rising up in front of the player could allow the player to cross over a path they previous walked along below.

The Build engine solves this problem by allowing multiple overlapping 2D paths to cross through the same 2D location. The key is that while inside one of these spaces, it is not possible to see into the other space, and so the 2.5D engine has no problem rendering one space or the other even though the map data literally shows the two areas to be occupying the same place. Several Duke Nukem 3D maps exploit this engine feature to create bizarre mind-bending maps where multiple large rooms all seem to be occupying one 3D space. One secret level in particular ("Lunatic Fringe" from episode 2, Lunar Apocalypse) has a map where the player can travel 720 degrees around a circle and not cross their path. This would be much more difficult to duplicate with modern true-3D map editors and engines.