Good and bad
This is a nice twist with a TD game where you can also level up. A like it as it gives the game a bit more replayability even after one has finished it.
What I'm really writing the review for is that, as mentioned by all other reviewers, the ninjas are way too overpowered. I mean come on, not only are they the fastest? One of the fastest units in the game but they can also turn invisible too? How is this fair at all? The ninja boss goes invisible right at the entrance, grabs a gem then reappears as he's about to exit. If there was a tower or even an upgrade(which would be so unfair at early levels even if there was) that could help you spot ninjas, it wouldn't be so unbalanced but hell, the times I lose, it's always the ninjas.
This would have been a 10 but I'm giving it a 7 because it impossible to get excellent anymore at the latter stages when the place is swarming with ninjas on hyperspeed.