Incredible
Really, really good. Instead of focusing on all of the great aspects (which there are too many to point out here), I'm going to point out the one thing that may need a little change in the hopes that this issue will be patched: the ninjas. I would suggest giving them a little more life and making their invisible sprints shorter but more frequent. So you'll be punished for not having towers in a variety of areas, but the way the game is, the ninjas can completely by pass all of your good towers. In many of the levels you're encouraged to have a cluster of towers near the entrance (or some other choke point) and I find myself completely changing my style when considering the ninjas. But besides that, this game is just incredible. It's very rewarding; especially the 'Skills' mechanic. Some games try to implement a long term rewards system by adding trophies, but with this game, multiple runs actually affect the game-play. It's just so great.