There's something with good potential here - I like the idea of building your own dungeon then facing the consequences of that build. Though you'd want to balance an easier escape versus greater reward so the player has strategic choice weighing up pros and cons.
I did find the controls a bit unintuitive. Tab for build mode should be displayed on screen at all times, basically - it's that important. It would also then work as an indicator showing you what mode you're in. N and M to move the build positions also took a bit of getting used to. Perhaps zooming out in build mode would free up the WASD from camera movements and allow more intuitive placement controls. Finally, E to open doors felt unnecessary - just open 'em up automatically.
Otherwise, I think this is a promising POC! Loved the visuals and atmosphere. Any Blue Prince inspiration? I really loved that game and look forward to seeing its influence on new games.