MAP18 - DEAD MAN'S TOWN
I really liked the opening area, then disliked the middle part (too many chaingunner snipers, cramped areas, etc). Once things were cleared out a bit it was more fun with finding the blue and yellow keys. I liked the Arch-Vile placement, too many people use AVs alone when they're really much more of a challenge as part of a group of other monsters. Overall my main complaint with the map was waaay too many Barons. It was probably a bit different back in '94, but mouselook makes them one of the lowest-threat monsters at any decent range, and they becoming nothing more than an ammo sink.
MAP19 - MALTRAITER
A nice, easy little romp after the overly-Baron-heavy last map. Most of the enemies here are zombies, imps or demons; only a couple revenants, mancubi, hell knights, and a caco and a baron to round things out. Not too shabby, but nothing too memorable either.
MAP20 - MOUNTAIN DEPOT
If nothing else, I can definitely appreciate the general design of the level - having a large, bright outdoors area was a nice change after lots of dim, indoor, cramped areas. Also liked starting in the middle and being able to go through the doors behind or through the doors/bridge area ahead. Still, I wouldn't have filled the outdoor area with lost souls, hitscanners and revenants... quite annoying. The cyberdemon fight is nicely placed and has an invuln sphere secret nearby to take the edge off. As for the red key, yeah, bad design, but the rad suit and easily visible red key was enough to get me to jump in and spacebar everything. Would've been better with a visible switch, I think.
Couldn't figure out how to get the BFG, though (got the doors open somehow, couldn't get the platform to lower).