While Doom looks 3D, the layout of the maps is actually 2D. Even that is not entirely true, it's sort of 2.5D. For any (X,Y) coordinate in Doom, there can only be one Z. In Doom, you will never walk over a bridge that you can also walk under. They will instead use tricks, like moving the floor up and down. Quake and other newer games are a different story.