Wonderfully atmospheric, and perpetually unsettling.
I will throw this on my steam wishlist, but what I wanted to say for now is that is that you did very well.
I think is a post "Portal" world people understand that puzzles can have their engagement increased by form and function coexisting, and I think you nailed that.
You took a risk that was hot debate in the 90's Dev team scene which if a demo can lead to sales, and as a fan of Serious Sam, the demo and the main games, and Meat Boy's flash version, and it's wider release, I think you have won that gambit.
I'd would almost scale back to fewer levels, to leave people more hungry, but I know there is an economic balance of social good will in being so forward.
Thank you for making this.
(edit)
PS I love your comittment to hiding things. You found some really clever ways of doing so.