> For instance, Quake was the first FPS where the maps were truly 3D and could have one piece of floor on top of another.
And you could see both at the same time. The Build Engine used by Duke Nukem 3D could also have one piece of floor on top of another floor (or even going through another floor), but due to how it worked, if both were visible at the same time you would have a "hall of mirrors" effect instead of rendering properly. And also water: while the Build Engine faked it by teleporting the player to another room when entering or leaving a water area, Quake did not need that kind of trickery.
Other than Duke Nukem 3D, other popular FPS games from that era also used "2D map with varying height" (like the Doom family), or even simpler things like rectangular 90-degrees-only single-height maps (for instance, Wolfenstein 3D).
And you could see both at the same time. The Build Engine used by Duke Nukem 3D could also have one piece of floor on top of another floor (or even going through another floor), but due to how it worked, if both were visible at the same time you would have a "hall of mirrors" effect instead of rendering properly. And also water: while the Build Engine faked it by teleporting the player to another room when entering or leaving a water area, Quake did not need that kind of trickery.
Other than Duke Nukem 3D, other popular FPS games from that era also used "2D map with varying height" (like the Doom family), or even simpler things like rectangular 90-degrees-only single-height maps (for instance, Wolfenstein 3D).