Nice work.
Easily the all-around well-crafted defense game on Newgrounds, if not in the entire flash games community. The physics engine is outstanding, and probably is what separates this game from the rest the most. While the gameplay is outstanding, the lack of a storyline in "campaign mode" is a little disappointing.
My strategy(For campaign) is to only upgrade my weapons to get the second level(pink ball). I upgraded the rest of my stuff from there, and it worked well.
Congratulations on the success of the game and I hope you do well on other work. (Although you did set the bar pretty high for me because of this)