This is a very good game! I love the aesthetic, the discovery progression, the monsters, weapons, and grim setting.
I love "ye olde" Doom's aesthetic, and this game captures it very well in a small space. It has a good balance of difficulty also. I love how one is able to get actually lost, yet it is still small enough that you don't absolutely require a map.
A few minor suggestions I would make would be to have a music change on the boss level; the ordinary level music seemed incongruous, and the music change earlier had a very good effect. A more dramatic sound at the death of the boss would be good.
It's the smoothest online FPS I've seen, one of the smoothest online games I've seen. It's the kind of FPS I would like to make.