There should be a "Restart Wave" button. It's very frustrating getting to the last wave, having 300 enemies coming after you, and having one mistake force you to start over. That's not difficulty, that's frustration.
This game makes you seem very selfish. You're using and throwing away people all for your own personal gain. Of course, it could just be a game about spinning squares.
It can get a bit boring and overdrawn. Once I have set up some troops and traps, nothing can kill me. 20 Waves and upwards? I can barely make it past the first couple without going to sleep.