WC3 Tower Defense in flash!

[Angrist]

For anti-air you build a crapload of air towers, which you sell again before the end of the level. I don't like this strategy though.

[thatmariolover]

I think the difficult part comes in between 30 and 35, when you have to start systematically replacing most of your old towers with the elemental towers (and then again with the combo towers). If you can get past that hurdle successfully it gets a bit easier again.