Really cool level and game concept. I like this games where you have several waves of enemies and you have to defend. As of the game itself it works fine, the music is pretty good, I liked the possibility to upgrade towers, link an element to it and even combine elements.
At this state, I think you need either add more different levels, or increase the game difficulty with each wave. The game itself is nice and beatiful, but it's boring because there is no challenge. Once you position the towers, that's it you've won the game basically.
Keep it up because the game concept itself is good, and the game works. Now maybe just strive towards making the gameplay more fun by means of being a bit more challenging.