I particularly liked the blood splatter when you defeat enemies - it's a nice subtle touch, but adds a little feeling of 'permanence' to the game. Although I enjoyed the music to begin with, it did end up getting a bit annoying - I think you could have a 'turn music off' button, or maybe add some new music so it doesn't just loop the one piece forever.
Overall this was an enjoyable top down shooter game, except without the shooting, and the controls felt very responsive.