Pretty Good for a 10KB Game
I'm not a hacker. But I would still think it is difficult to come up with a good game using fewer than 10 KB of code. Given that restriction that you put on yourself, you made a playable game. The concept is not terribly original. I know I've played games with this same strategy before. But you put enough challenge into the controls of the game to keep it playable through five levels. And that was a good cut-off. Anything over five would have made the game feel redundant. Even as it is, there didn't seem to be a huge increase in challenge from one level to the next.
Since you put yourself under the 10 KB restriction, I am not really sure how to offer constructive criticisim. Under such rules you can't have blowaway graphics or sounds. In keeping with the theme of the game, if it were possible to put more variation in the enemies, or bring topography more into play, or give the player a limited amount of shots... those sorts of things would up the challenge.
As is, the game is a solid effort and good demonstration of what can do with such a limiting restriction.