Fantastic visual style! I love the low-res sprites with fluid animation. In terms of gameplay, though, I felt like I was missing something--on every difficulty, all I had to do was stand inside the boss and spam attack to win, and I didn't see any "spin" involved except for the spinning axe. After I read the comments mentioning the room spinning, I slowed down my attacks and finally saw that part of the fight. Very cool idea once I saw it happen! Love the way the scenery changes...but then by the time it happened, it only took a couple more hits to kill the boss, so I didn't really get to experience the changed room.
You've got a great concept here, and you've absolutely nailed it on atmosphere, it just needs some tuning.