Ok, just played this. Have a bit to say...
ok, so I made another quick map for this one:
inc.wad... I think I was going for incubator, not sure.
its a tek level based on what I would guess happening after my first map.
I got to admit, the last parts pretty lazy.
it has difficulty settings of:
easy (I'm too young to die/not too rough): a map based around my playstyle.
medium (hurt me plenty): a map designed to challenge me on how I play and actually made me switch my style a bit
hard (UV/ nightmare): a pretty challenging map (for me) that contains a lot of monsters
its in the map02 slot, but I could easily change it
tell me what needs improving and I will get on it
First off, played on UV and had to give myself a total of 170 extra shotgun shells via the console throughout the map, because there's just way too many monsters and way too little ammo.
Second off, in the earlier parts of the map there are a lot of demons/spectres, and many of them are placed in a way that they can easily be avoided. After you teleport, there's like 20 demons waiting for you, but they can't reach you so you can just snipe them off with ease. The same thing happens later, with the spectres down in the area below the stairs, with the imps.
Third off, monsters. Way too many monsters just placed around randomly. This would be OK had the map lended it's design too it, but it doesn't really. It's just a series of relatively tight corridors and rooms interconnected with each other, and behind each door there's a big baddie. So progress is a slow matter, because no matter where you go, you have to fight off some mancubus, or barons or pain elementals. Most of them are relatively easy and (sorry to say) tedious fights because they are all in the same scope: In tight corridors.
The mancubus fight with the three mancubus on pillars was good. That's where they shine. Keep those. Remove all other mancubus in the maps. Also, try to keep it a bit thematic. In one room there's a revenant, hell knight and mancubus, just standing around "waiting" for you behind some corner. Make them all into revenants.
Also, in the room behind the red door, there's like 8 - 10 mancubus. Remove them all and replace with lesser monsters. Shotgun guys, chaingun dudes, imps and demons. These are monsters that work best in corridors.
The same goes for the legion of revenants and lost souls. Also, try adding a few traps in the corridors. Keep you on your toe. One revenant in a trap is worth ten behind a corner. Make them suddenly pop out of inconvenient places (for the player).
Fourth, in the Mancubus fight by the pillars, remove the partial invisibility spheres. They're not much good. Replace them with something else, like ammo.
Fifth, the weapon room... Remove it and make it into something else. Everything in this map is rather flat and cubic. Try making it slightly bigger, with the prize waiting in the middle (let's say it's a soul sphere or plasma rifle) and design a way for the player to fight to get to it. Doesn't have to be a huge room. But what if the sphere is on a pillar, and you have to press four switches for it to get within reach? All the while, also triggering monsters?
Lastly, the map looks very clean. You did a good job with texture variation and a nice cleanup. I can't think of much visual to point out. Maybe add some light variation where you feel it's a bit flat? Also, expand the first room beyond the starting room a little. It feels cramped.