Nice graphics & sound, but I get the feeling that they're ripped off of an NDS game. Aside from changing the art for the turrets and explosions, I couldn't figure out what the turret upgrades were doing. Having 3 basic turrets was better than one because you could hit more enemies, but spending another 1000 (currency unit)s to make them uberturrets... did nothing. At all. As far as I could tell. On the plus side, it did give me something to spend my (currency unit)s on.
Other than that, my only complaint is that the levels loop, so it isn't clear that you've already beat the game and there's nothing new to see.
Since there aren't any purposeful upgrades, and there's only one turret type, this is basically a run-of-the-mill click-to-kill defense game. It does have the timing mechanic, where you have to fire at the right time, or your towers don't do any relevant damage to most enemies, but this in itself detracts from the game as the lag builds up. Haven't we been doing this since PICO came out?