Since some enemies are shielded against specific types of ammo, you are forced to use different types of aircraft to attack. This not only adds to the challenge of the game, but also motivates you to earn Gears and buy new units, as quickly as possible.
What I love about Air Patriots is that it strikes just the right balance between unlockable game attributes and purchasable game attributes. I've encountered too many "free" titles that let you taste maybe 10 percent of the game, and ask you to pay (with real money) for the other 90 percent. Air Patriots doesn't have that problem, as it offers plenty to discover before asking you to whip out your (Google) wallet.
As great as Air Patriots is, though, the game is not perfect. Some greater variation in enemy types would be a welcome tweak. As of now, there are only four different tanks to contend with, which can get repetitive after a while.
Overall, if you're a fan of traditional tower defense games, with fixed paths and stationary turrets, then you'll love Air Patriots. Its mobile units and looping attack patterns add a whole new dimension of challenging gameplay.