The funny thing is that I didn't care much for this game, but I found myself playing it. You probably could have made it better by putting in more detail. It seems like it's just the same enemies being shot over and over. Of course, it still isn't terrible. The best thing about it is probably how you have so many cool upgrades. Yeah, it's still a tad annoying with how they all go away with one death, but still fun.
I thought the music was nice and fit the mood pretty well. Everything is in fact pretty easy to understand, I guess. The background's alright too. I appreciate you having some fairly good graphics. It's okay, I suppose.