This was really fun. I started a map, scrapped it once I understood that it wasn't a straight map, started a second map that became more and more convoluted. I ended up being really confused around the yellow altar area and abandonned the map again since I had a rough idea of the sequences to finish but was to lazy to reorganize the map...
It was an original idea and I wouldn't have thought that was your first 3D game.
It's just missing some music. Doors and footsteps sfx would probably add a lot to the ambiance of getting lost in an empty building.
I disagree with the people wanting an in-game map though, I think it's good like that. Putting your hands of the keyboard to look at your map wondering where you are and then finally connecting things together is more fullfilling for me.