Not bad
Gatta love easy mode. For the tons of people who think your god on easy mode, once you hit about level 60+ (which I did, I was so bored a few days ago) the planes get so dang strong that it's not even worth having anything out. However, you can live almost indefinately (I got bored around 70 and let them kill me). By then, you should have all your upgrades purchased. All you have to do is keep shooting down the enemies with your tower, and putting up force fields around the center right of your tower. The jets cant shoot through it. As long as you keep shooting them down, on that strength of enemy, your shields will recharge nice and fast so you can keep using them. Also, spend all your money on Health.
Anyway, to the actual review. The game was great :) I loved every second of it. It's one of the few defense games that, to me, scales appropriately from very easy to damn near impossable. Make another one with even more options. Just...maybe offer some more music tracks, and better sound effects. After 60+ levels, they really, really get old. Some different enemies might be nice to, but the ones you have did cover what was needed.