Well done!  The fundamental game play is solid, and I really like the art style.  However, the game is really difficult, which I think is due to the slow fire rate on the pistol.  I might just be playing it wrong, but I found it almost impossible to kill the robots consistently because your pistol shoots at the same rate they do, meaning they just block your bullets with their own.  I also would suggest allowing free movement up and down the y-axis instead of limiting the player to three spots on the screen.  All in all, really impressive for the amount of time we had!  I would love to see an extended version like you mentioned on your game page.