Doom - or really a lot of early FPS games - the levels just tended to be a bunch of random mazes, done up to look like they're actual places. Doom in particular, though, because it was advanced enough that the level designers could add quite a bit of detail. In earlier games like Wolfenstein, yeah, you could only design levels on a square grid, which severely limited the kinds of levels you could design. However, Doom allowed for the possibility of archways, stairs, windows, walls jutting at any angle, skylights, etc. However, when you get down to it, while the levels look nice, they're just mazes, even though they're supposed to be representations of real-world places. Why does a base on Mars have all these random corridors, walls at random angles, passageways that lead nowhere, etc.? Doom 2 really takes the cake, because the second act takes places in a city, and while it certainly has a city-type aesthetic to it, it doesn't look like a real city - buildings are haphazardly strewn about at random and make no sense, there are no streets, there are all kinds of randomly placed stairs and lifts, the interiors are mazes... It's like, if this is how people were designing cities, they were doomed well before any demon invasion ever took place. Again, I know part of that was the limitations of the engine, I just find it funny, especially because when I design my own Doom levels, I always take into account, "Okay, this is a real place, I have a certain layout in mind for gameplay purposes, but how do I logically make the level fit the layout so that it looks like this could be a real place?" So for instance, I'll add fake doors to indicate there are other places you could go to if they weren't locked, I build it so that if you look out the window, you can see other parts of the base that aren't part of the map, if I have a cargo area, I make the doors big enough that the crates inside could actually fit there, etc.