Wow!
I just beat this game... The 6 levels (On that other site though) and I can tell you that this was the most challenging tower defense I have ever played (flash tower defense that is).
Once you understand that you don't need anything else than the rocket towers, depleting and ice towers and anti air towers... You're good to go...
The hardest was to build the maze WHILE bulstering your towers' power...
I loved the quality of the graphics and the sounds
The overall gameplay was kind of repetitive (once you lost one level, to start over again there's almost always only one strategy so it gets repetitive once you've lost 3-4 times) But the change of environment from a level to the other was nice...
The time required to master the game (and win it) is reasonable (took me 4 days to beat the 6 levels)
The learning curve is, unfortunately, too high... There should be some warnings (lets say three levels before) when an air strike is about to come.
You should make OTHER towers a little bit more useful.
A little tip for other people - Here's my "maze" for level 6 :
http://i107.photo bucket.com/albums/m282/Nass1022/XenoT actic2lvl100Win.jpg (remove the space between photo and bucket)