I'd recommend putting instructions on how to start the game (or add a start button).
Animations and pixel graphics are well done, very nice. I like the difficulty and variability in the enemies. If you add some background music I think it would really make the atmosphere come to life.
Text is difficult to read (especially pink). I was also able to run straight through levels without killing the enemies (unsure if that is intended). Also the boss knocked me out of the map (was able to walk back in the entrance).