It needs a lot of work on all the presentation aspects, BUT it's clearly playable -- with a distinct set of areas and an actual progression rather than just "monsters in a box" -- and it has some fun ideas and mechanisms along the way.

I feel like there's a lot of very specific bits of feedback that players can come up with by going through this map, like:

* Don't spawn monsters right in front of the player.
* If your map has zombies, don't put the only GL in a secret.
* Speaking of weapons, a NG would also be nice.
* Found at least one door that only opens from one side (and that doesn't seem to be a good/intentional choice).
* Speaking of doors, pay attention to door textures ... if it looks like a double door, don't make the whole thing slide away in one chunk.

Etc., but really at this point the best thing to do would just be make another map! Once you have your next map in a playable state, try to rope in some playtesters to give it a look.
Given the amazing content available in the community, this is likely to get some low ratings, but don't let it discourage you. If this is your first level, it has a lot of positives, but it's also a good place for you to reflect and learn a few things for the next one.

First, I liked the waterfall secret. It's a classic, and I always enjoy a good Dopefish.
The layout is nice and labyrinthian, felt like a good ol' classic Quake map. Nice work on the lanterns and chandeliers. There are a lot of nice little touches.
You had creative secrets, decent encounters, and you were clearly working to make each area feel distinct. A lot of promise here.

Now, some critiques. There are a variety of little things around the map that really show that this is a new mapper at work. No worries! But there are just a few big things I want to point out for the future.
1. I reached the end without finding all the secrets, but I couldn't go back to find them. Unless there's some narrative reason to lock the player out, always give players the option to return and explore.
2. Lighting all around the level is overkill and could be tweaked to improve the mood. I turned your lights off periodically, to make the level fullbright, and there was very little difference. Don't be afraid to allow some shadows, and spend time experimenting with light values like "wait." Lower wait spreads the light further, higher wait makes it hug the light entity.

In its current state, it's not a great map, it could use some tweaks, but it has pretty good bones. You can make huge improvements with more subtle lighting effects.
Thank you for playing the level!

My reason for locking the player from the rest of the level was that during playtests, players would fight the Hell Knights from the hallway before that and hide in there. I wanted to avoid that because it causes the player to optimize the fun out of the encounter and what I intended the fight to be like. But your feedback is definitely valid so maybe a different solution that doesn't hinder this type of exploration would have been better.
As for lighting, I unfortunately didn't spend as much time on it and was nearing a deadline, so I was unable to do multiple passes or iterate on them. That was simply bad planning.

I appreciate the feedback & kind words and would like to thank you again for taking the time to play.