Bloons Defence 3

This game is guaranteed to keep you busy for hours at a time. Use strategy to place bloons towers and destroy the attacking bloons!

http://www.ninjakiwi.com/Games/Tower-Defense/Play/Bloons-Tower-Defense-3.html

If you need a walkthrough, one is available at http://jayisgames.com/archives/2007/08/bloons_tower_defense.php