Great work, brings back memories and it's a win.
I say, it's like the era of Wolfenstein 3D when computers didn't have enough power to display floors and ceiling. You did a great job, I think though, perhaps using Q and E to rotate the camera would be easier than moving the gun to the far right/left. Also next time make doors open automatically. Nothing else to improve because it's great enough, I like it, enjoyable and .. it has a nice retro feel because of the 2D enemies and the whole idea. Keep it up! 10/10