I think this game is awful. The frequency is stupid. There would be times when there were no planes or anything on the screen for a couple of seconds and that was on level 14.
Nice concept (yes I drew something dirty) but expand on the gameplay a bit. Like avallozzi said, start with less money. Have more choice in upgrades etc.. Basically fill it out a bit.
Extremely simple concept but really fun to play. Strange cause I don't like simple games... Great art and animation but a bit too short, maybe stick in a few more levels and a pause button. Overall 4/5, good job raitendo.