Possibly the best tower defence game I've ever played. For starters, it has a plot. WOW! And a good one as well. With plot twists, and a couple good knives into the spinal chord. In a flash game. Mindgasm. Also, the game play is amazing. Strategic planning is mandatory for success. Most of the later levels require several attempts, and a lot of experimentation with tower placement and upgrades before finding a combination that works. Great variety in enemies, and the tactics required to beat them.
And easily the best aspect of the game. Well, the best two. There's no money penalty for selling a tower. Instead, any money you earn in game is stockpiled outside of your available cash and fed in at 1 unit/second. So selling things leaves you disadvantaged, but not crippled. And finally, all towers are customisably upgradeable. You buy a tower, and it has upgrade slots where you put benifits like splash, r.o.f., range and cooler things. If you like TD's i highly recommend this.
Graphics - 5/5 - quite appropriate given the game, and didn't lag my computer.
Gameplay - 5/5 - Best. TD. EVAR.
Replayability - 2/5 - After finishing a level, there's not much point going back to it. Unless you're a perfectionist. Like me.
Story - 4/5 - Entertaining, and some clever twists.
Overall - 8/10