http://www.ninjakiwi.com/bloonstd2.html
One of the best "tower defense" genre games I've come across (at least that is beatable).
I would start on beginner.
More hints (scroll down)
You'll need a cannon by around level 20
Upgrades are better than more units, but some upgrades are unnecessary.
The 8-way popper is kind of waste.
Unit placement is critical. The monkeys are right handed!
 
