I had a lot of fun with this game. I died the first time, but on my second run I made it all the way to Satan -- but died when he spawned into me. The random map is nice, and the upgrades feel good.
Three improvements I would recommend: (1) Full-screen please! (2) Give a warning where/when the enemies will spawn because it feels like a cheap loss when they spawn right into you. Maybe show some (non-harmful) fire or a pentagram for 1-2 seconds where they will spawn. (3) Give an indication when the character is hurt - like turn him red and/or knock him back a little bit.