I'm not taking about shapes, I'm talking about the LEVELS themselves. There are no DOOM levels where part of the level exists "above" or "below" another part of the level - they are all 2D maps that through various (ingenious) engine tricks, such as elevators that teleport you to fake entering one spot and exiting another spot "above" it, to visually appear 3D.

Hence my squashing analogy, DOOM levels don't overlap and can be drawn ("squashed flat") on a 2D surface.

But you can't actually occupy a different height for a given location. This is NOT real 3D. That's why DOOM is sometimes described as a 2.5D game. Quake was the ID Software game that was 3D.

If you think DOOM is a 3D game, try creating a level that contains a player accessible location above/below another player accessible location, and see how the game renders it. Or, read the DOOM rendering engine links that are posted and pay close attention to the "not a true 3D engine" paragraph.

That is a pretty arbitrary qualifier for 3D. Doom is filled with spatial obstacles that make use of the 3D geometry. For example, a missile can pass over the player. Facing a wall on top of which a monster stands, I can't necessarily pass it or see or fire at the monster. Flattening the levels would change the game and remove many of its challenges.

The format of the levels or the fact that there are no rooms over other rooms don't really factor into it.

I think we're just arguing at different levels of abstraction here. I'm arguing that a 3D environment is being generated from 2D map data, and I consider it to be meaningfully 3D.

I suppose at the extreme end, I'd have to explain why Wizardry on the Apple II is or isn't a 3D game, which would be more of a strained philosophical argument.