The "Build" engine makes horizontally moving platforms possible by allowing several objects to pass through the same 2D space. While inside one of these spaces, it is not possible to see into the other space, and so the Byuild 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 use this 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. 