Really, the first "modern" 3D game was Quake. Doom still used sprites for all bad guys and stuff. IIRC, Quake was the first game where everything was 3d.Also, I believe Doom didn't support having one platform above the other (like a balcony, or going under a staircase). So like, you wouldn't be able to implement a multi-level parking lot (or even spiral stairs).