Very cool concept for a game, and a really good use of the theme/limitation. The execution of the dungeon spawning mechanic is pretty well done, although at one point I spawned inside of a block - I guess that doesn't really matter much because the character can shoot through the blocks, it's more of a nitpick than anything.
I will say the sounds did layer on top of each other at points, so the game would randomly get pretty loud - I would recommend preventing more than 1 instance of a particular sound playing at the same time, for example, you could have it so that before a sound plays, it stops all instances of that sound which are playing first. Not sure if that's possible in Godot but maybe it's worth looking into?
This is an idea with alot of potential. I don't know if you've thought of going further with it, but I'm sure you could develop it into something really special. Great work ^-^