Interesting level design with choices and twists. Nice looking sprite for the main character, good music. Skull mode is adorable!
For an game region of this size, a map would be really helpful.
Also, in my opinion, the little devil-heads are a little too tough to be the most basic enemy type. It makes it very hard to traverse a 3-screen area without suffering any damage.
Lastly--this is a small thing, but the attack animation of the main character doesn't look very aggressive. It almost looks like he's laying out a tablecloth. :)