Ninja'ed my 15 minutes, plundered by laziness
Yinan my header may seem a little harsh but hear me out. This game is a TD and the genre has been pretty well set out as far as quality goes. Unfortunately it lacked on a few levels.
You had the seven or so towers to choose from, and a few maps but it was more or less the same. For a TD it didn't offer a lot for me. If anything it would be slightly under par for TD game play compared to what we have seen on Newgrounds and other flash gaming sites. This may be a first attempt for you and if it is, I'm sure we will only see you rise from here.
Some notes to improve upon. You have the ability to upgrade towers which is a good thing, and they are clearly labeled what level they have reached. Perhaps changing the graphic for each level, or at least every second level may have feel a little more fore filling. Example, starting off standard ninja, then perhaps a boost in size with a combo of uniform change, to finally morphing into a highly menacing entity for its end level.
Medium also seemed very simple, more challenge is required. Having finished all waves I was then treated... nothing. I'll understand if you do not have the technical capabilities at the moment to send a players score to a central database to compare with other players, but a screen to reward our efforts in completing the game would have been nice.
After all that being said, I still had a few minutes of fun and the graphics were cutesy. I'd look forward to seeing what you may produce as far as TD's in future, and hope you wont be afraid to really push the boundaries and perhaps do a little research into what some of your other competitors are doing with their games. Thanks for the entertainment.