In my opinion, this game has the most consistent art style and theme. The music isn't bad either. Making a tower defense was also an original idea. Sadly, a very easy game to beat : I placed a single tower on the last level and upgraded it without any worry.
It would probably work on balancing the game a bit more and as the other comment said, add more enemy diversity. Bloons Tower Defense is a great example of a TD with challenging diversity. Cheers :)