I don't know if you are continuing to develop this, but you have some very solid gameplay mechanics here. I have played about 50 tower defense games and this has a lot of promise to being a great game.
A few comments if you are still looking to develop this game further:
1. As stated by others, music is too loud & too generic. Give option to reduce or turn off.
2 Have more sound for towers firing & enemies.
3. Enemies that stand out from each other. Right now, each one looks like a block with hit points. You need more variety.
Anyway, I'd love to play more of this and give you feedback if you decide to return & polish this up a bit.