Simple, and a bit bland in style, though the gameplay was smooth and it was definitely worth playing. Difficulty curve was alright, actually. A few design elements were questionable, but then it was for Ludum Dare.
The stark black and white hurt my eyes, especially when I came up against walls and it was just a large expanse of pure white on my screen. I don't know if you could do less contrast, more color, but just telling you what I felt. The music was very repetitive, and especially the trill upwards is a bit grating. If you're going to have a main theme and one that is fairly peppy, it's hard to use midi sounds simply because no matter what they have that shrill quality. A calmer soundtrack or a more varied one may help.
I like the musical theme itself, it just got on my nerves after a while as a constant thing in the background.
I'd just point out that medals are typically awards for trying something out of the ordinary, with maybe a few 'checkpoint' medals, instead of one per level. One idea is beating level one without using a gun, which is possible. I only found this out because I didn't see that Z was shoot until I got stuck on level 2 and scrolled down.
The boss was well-done considering the few resources you had in the game.
Overall I can't call it amazing, but it was well-done and quite clean of a game. The biggest minus point for me is definitely the bright white and its effect on my eyes.