Liked the idea. I think there are so many ways you can fill your game. Like auto guns, new weapons, shields etc. Maybe it's because for me it feels like a bit of good old tower defense experience. I liked it. I was also suprised (like other folks below) when graphics changed, thought it was a bug, but mmm too smooth and well made for a bug :) I'm sure if you had more time you would have made it much clearer. For example, I would like the bullets to fly much faster (like tratatatata and BOOM BOOM feel, I hope you understand)