Tower Defense Game - any ideas?

My 8 year old son wants me to help him make a Tower Defense Game, like bloonz by Ninja Kiwi.

Does anyone know of any examples here, or where to start? We just downloaded the program and ran through the tutorials, but the world editor is really difficult to use.

I think a basic "top down" view would be best, but I don't know how to do that.

Would anyone be interested in joining us for this project?
Infinitum3D@hotmail.com

Thanks!