Killer Idea
Well, I would say that this is a great idea for a much, much larger game.
Multiple enemies, power ups, larger screen, smaller details, backgrounds, sound effects, and the same Muse (:)!) and you got an awesome game.
Really, it is fun to begin with, but as many before me have said, the game should get harder, not easier. Speed up the spikes when you level up.
Finally, you've got some talent. Keep up the good work, and we can start expecting some wholesome goodness in the portal soon. I gave you a 3/5 for creativity.
--jmtb02