Since each level offers new enemies and towers you will have to keep changing your strategy, which is great because repetition is common in most tower defense games. (mashthosebuttons.com)
To me, I usually see most tower defense games as time consuming and repetitive. (darkstation.com)
Basically like with most tower defense games you start out with cheap units and get progressively more expensive and more powerful units as the game goes. (droidgamers.com)