Well, everywhere needs some detailing. The idea of first level is pretty nice since it's not done I didn't do that much. A bug I noticed is that if a marine gets hit, he will stop patrolling around. It might be the limitation of ACS but I don't know.
Second level is good. A door needs a yellow key instead of red, and it has red key walls next to it... Other than that, the area with crates can use some ventilation system above it to add some detail. Also the hallway with 4 different paths. Use some lighting there and add some broken ceiling/floor etc. There's also a misalignment at one of the toilets. The elevator which goes to dungeon has a logical fault. Let me show;
IIIIII
I
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That is the elevator of second level. And this is the;
IIIII
1
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Elevator of the third level. The elevator should have two entrances at both M2 and M3. That was all I've noticed.
For M3, the area where the hell knight comes is pretty good made, you can leave it like that. However, I didn't get what to do when I went in where the Hell Knight came to prevent myself from getting crushed. I was just stuck over there...
It is a bit patience a bit skill and a bit experience which comes with patience. I suggest you to check them. Also, making slopes add great detail as well. How do you think they have become so great mappers? It's with patience. Trust me, if you can try a bit harder, you'll be successful.
Also, you just click the users name and view public profile in order to get to the message area >.<
Oh, believe me you can add many details to doom. Well, I'm sure you are already aware of the mods that Tormentor and the community had made. You can always check them out for some nasty details.