When I noticed the map didn't have a lit file, I got excited and decided to play the map on the "oldchool" winquake mode of the Mark V engine, so I'm not sure if the demos
(first run, skill 2, 100 %, one death, around 14 minutes) will run on Quakespasm.
I agree with Poorchop's compliments on the mechanic of keeping the player moving with the zombies, that remain unkillable for a large portion of the map, until he or she find a GL/RL.
As you'll see in my demos, I was able to "break" the map by being accidentally able to come up for air in the water pool, which is why I was just barely able to survive the pool. Poorchop's comments were pretty much spot on on the place. I, too, ended up wasting time trying to secure myself a breathing spot, instead of just concentrating on finding the shootable button. Maybe you could have some bars shut the way above the player as soon as the player is flung into the water? Then it would be more obvious for the player to not even attempt to go up. Maybe also add some health pickups to give weak players some health to drain while running out of air?
I liked the aesthetics of the map, which was an interesting blend of medieval and base. I think the mix worked really well! I think I liked the underwater ruin area the most, though. It was brimming with detail and the large number of air bubbles somehow just stuck out as a nice detail in particular.
I think the secrets were pretty nicely done, too. I'm not sure if I would have survived with only one death, though, if I hadn't found them. Also, had I played on nightmare, I probably would have died more often. I was actually going to play on that difficulty, but accidentally had chosen hard instead, which may have been the right choice after all. xD
All in all, a really nice first map. Good work!