Quake and the Z axis

The original 3D computer games had one small detail in their game engines that eventually would need to be solved and that was that they weren't quite 3D. Full three dimensional freedom of movement means that space exists and objects can be located in any of three axes: two horizontal and one vertical. But in the original DOOM and its sequels, there is no true vertical axis. This was done partially because of the limits of slower CPUs and partially because of DOOM's engine design.

Fortunately the game was not all that limited by the reliance on only horizontal location. The Z axis in DOOM was handled automatically. It was not until the original Quake that true three-axis location and trajectories were possible, and once again, the hardware requirements increased. Quake was the game that brought about most, if not all of the original Pentium upgrades, because true 3D math takes a lot of processor cycles.