It's an enjoyable enough base for a decent FPS. I did manage to make it to the exit with out any major difficulty. I did unexpectedly die when checking out the furniture in the room you start in, as I didn't realize there were enemies outside. (I find it weird that there's no collision for the furniture)
I like the fact that the gun is a piece of cardboard, but it would be really cool if the gun was completely made of cardboard instead of just a picture on cardboard. (This idea might be asking a bit much though)
The idea you have for secrets is neat, but right now they're just somewhat of a disappointment.