I enjoyed it, many games people don't understand how to make the difficulty ramp at a reasonable pace. I think you have that down, you get just far enough in to learn it and it gets a bit harder, but not super hard. You also know how to keep it interesting with new skulls, i was wondering what the next new one would be. I enjoyed it, if i had anything to say bad.. (and maybe there is some farther into the game) maybe some kind of power up for player.. even something cosmetic (like your bonus items matching the skulls, nice touch) so when you killed say 100 skulls without damage your ship would do a loop.. Since actual upgrading the power of the shells would mess with your game mechanic the cosmetic "bonus" things would enhance.