looks good, but the arrow keys cause the browser to scroll up and down. easily remedied by going full screen, but it's something you should look into anyway.
it's very easy, you can destroy enemy projectiles instead of having to dodge them. since there's no limit on how fast you can shoot, the boss is trivial. most enemies are trivial too, but you'll get more creative with time. music is inoffensive (love me some MMBN music) and at a good volume, but it sounds a bit out of place.
best of luck on your next project. did you do all of the artwork your self?