Here's a Skill 1 demo with commentary throughout.
Things I liked:
Nice little old school map. I prefer shorter maps so this was great for my tastes. The secrets I found were fun and rewarding. I liked how you wove in some outdoor areas here and there. I'd suggest adding more if you want. i.e in the the silver key area you have an enemy fire in at you from outside. That was cool. The exit room was really nice. High contrast looks great in there. You have some nice multi-stage contraptions (silver key room) - more of those would be awesome.
Some things to consider:
Base maps are kind of hard to nail because they can get "busy" very quickly if you use a bunch of colorful textures. You have some very "vibrant" areas in the map that are almost too much to look at. So for example you have some doors with big yellow light textures surrounding them. Some of those are a bit much for my tastes. But this is a subjective thing. I think overall you have a decent looking base map here.
As far as gameplay. It's an easy map.I am going to go back and play this on hard to see any differences. Many of the enemies are on the same plane as the player. That's too Doom-y for me. There's one area where you have enemies attack from above though. That took me off guard because it was rare in your map. You may want to mix up some of the elevations in the map where possible.
You had some dead end areas and that's okay but I was a bit confused here and there as you'll see in the demo. Also maybe mix it up a bit on your doors. I missed the nail gun area completely. If you had a "see thru" door that showed me the gun was there I would have noticed it. As it stands, you have some doors that are unlocked and some that are "opened elsewhere.
One last thing: those "phat" railings in the central slime room are out of control! I'd re-work those to match some of the better railings in the map. Makes for much better visibility. I didn't notice the hatch until much later for example.
There are some buttons you need to fix the lip and add wait -1 after they are tripped. Little fixes here and there.
Okay, that's enough from me! Good job. Keep mapping and enjoy yourself.