You played my game so you know I struggled with a similar thing (not that I did any better)
The first thing is yes - I think rooms should be bigger. You could also try to "break" them apart. I tried adding columns and walls inside the later rooms in my game (you might have them as well but I didn't see them in the part of the game played). Also 'L' shaped corridors tend to help, I noticed
What I tied was also to activate enemies only then they see the player (and the player, by extension, sees them) - it doesn't work perfectly in my game because it was literary the last thing I've added :sweat_smile:
Door sound (and screenshake?) definitely would help a lot
"Maybe closing the doors clearly with a sound first and then having a couple of seconds delay before the enemies spawn? During the wait time the exact spawn position could be indicated with some kind of effect" yes and yes! And please add a color to the effect so that player knows what kind of enemy is coming (I think this kind of ahead of time knowledge adds a ton to a combat - it allows you to plan and then try to execute your plan)
When I was making my game (again, not that I succeeded) I tried to give player 2 things: at lease a second or two of a warning (as in the player sees the enemy and has some time to think before the enemy is too close) and the ability (the space) for the player to move around
Sorry, it ended up being a long answer :blush: