Graphics are pretty solid, but perhaps in your level design, avoid placing the enemy VERY nearby the door as this would count (for me) as a bull-headed move from the designer. The difficulty is similar to every other platformer in the world, so you got nothing to worry about for that one.
Though, you should try to make the spikes pop up more with a shine animation on them or something.
Just a nitpick here, in the credits, the labels are not centered! >.<