Way too difficult
I passed all six levels on the free-online-games-directory.com site. 4 & 5 were actually easier then the first 3.
The problems?
-Way too difficult
-Bad range of towers
-Bad upgrades (Speed is useless, range somewhat)
-Poor freedom of strategy / no variability in strategy.
-Why do Air units move from top to the bottom, when the height of the maps are small?
The only way I passed the game was by 1) buying SAM towers and upgrading them to max damage, 2) mazing. The last level's last wave was insane and long.
The game mechanics need to balance, and work! Till then, it's mediocre.