Alright, just got through it, sorry about the delay (I've been inebriated, asleep, or hung-over since you sent it). Here's some stuff I noticed:
This shadow doesn't look right... Not sure how you'd fix that, but there ya go.
It's not immediately clear that this is a lift, especially if someone was playing without freelook and couldn't see the key above.
This door isn't lower unpegged. For that matter I don't think any of the doors in the level were, though they weren't using the usual doortrack texture so it doesn't look too odd, but I question the choice of having the same texture for both walls, the ceiling, and the floor here.
This poor demon can't fit between the crates and the trees! It doesn't make him very threatening. :P
More non-unpegged doors! Again it doesn't really look that bad, but it's always kind of jarring to me.
I dunno what was up with this room. The first time I went in I got trapped inside for several seconds for no evident reason, then it let me go. I decided to explore the room again a bit later and got trapped again, but this time I was never let out. I shot and activated every inch of that room to no avail, so I think you need to take a look at that room. There's no enemies in that room or that spawn outside, so why does it even trap you the first time?
I dunno how many maps you're planning to make this, but I think after finding everything in the map the player is really kitted out for just finishing MAP01. I headed to MAP02 with 100 Health, 200 Armor, full bullets, full shells, a chainsaw, and a plasma rifle. If you're planning to make this a longer WAD that seems pretty early to have that many goodies. Also I managed to get 100% Items/Secrets, but I somehow missed one single monster... Odd. Also things were a bit on the easy side playing on UV, but there's nothing wrong with that. It's not like I could have finished the map with my eyes closed, so it's challenging enough to keep things interesting! :D
Other than that, map looks really nice! It's maybe a bit claustrophobic, but I felt like that was part of the theme so it worked. Looking forward to more, hope I helped out some!