Suppose you're a Doom designer, and you want to create a corner. There's a couple of simple ways to do this. You might put down a vertex, marking the corner, then draw two lines from it. Or you might create a line, split it in the middle, then move the resulting corner where you want it.
Whereas if you're the author of this level, what you do is create two entirely separate lines, move the vertices at the ends of these lines as close together as you can (although not actually touching), then go "Ah, good enough. Onto the next bit!" And repeat, over and over. This is a level in pieces. Almost none of the rooms here are closed geometrical structures. Look at any corner and chances are you'll see a thin vertical line marking the spot where the author failed to fill in the geometry. You're literally staring at nothing. Frankly I'm amazed that Doom can even handle a level as poorly structured as this one, not to mention record a demo on, or for that matter play back in a sourceport (and it's possible to do all three).
If that were the extend of the level's flaws I guess it wouldn't be remarkable. But the author shows a diabolical genius throughout for offputting effects. Like the many glaring HOMs... the moments the level suddenly drops away leaving you staring at nothing... the multipatch texturing errors... the teleporters that only work when you run backwards across them... the zero-thickness walls... the bizarre sargeant encounter that I think has something to do with the fractured geometry but I'm not sure. I'm still not sure if any of these was deliberate or just stumbled across accidentally.
I could have sped through this level a lot faster, but you'd miss most of what makes it remarkable. So here's a more leisurely run, done in Dosbox so you can see how this would have looked on the original game, with annotations pointing out some of the level's more notable features.