doom's rendering is 2.5d (but is still fully accurate to 3d rendering, if the pitch of the camera is fixed to 0), but doom's _logic_ is fully 3d (the only reason enemies were infinitely tall was to save on cpu speed (and different to enemies, projectiles always had full 3d collision), it was just a few lines of code change to make them have real height for player-enemy collisions, which was done in heretic, another game using the same engine as doom)