I particularly liked your use of foreground and background elements, so that I had a nice parallax effect of the hills in the background, but I could also 'hide' behind the trees - it gave a nice sensation of 'space' to the game. I also liked the 'spread' on bullets - a nice subtle touch.
One thing I think I would have liked to have seen would be your scenery being drawn in as much detail as your main characters; they are at a very high resolution and really 'pop' on screen, but then your trees look quite blocky by comparison.
Overall this is a fun little shooting game, and I hope you continue to work on it with new levels!