Took me about 30 plays, but finally beat all 50 levels. One of the tougher tower based strategy flash games in my opinion. Big trouble getting past level 23, then once I could do that I could get to 43 and stalled out again.
http://www.addictinggames.com/bloontowerdefense.html
http://www.addictinggames.com/bloontowerdefense.html