The art style and the music are both awesome! I think you have a really good basis for a tower defense game in here, but there are a lot of things that feel like they're missing that kinda hold it back.
Without a wave counter or an end goal in sight, it just becomes monotonous. I set up a bunch of turrets and eventually I was just sitting there watching them kill things for several waves without having to do anything. I couldn't even tell if the waves were supposed to be getting progressively harder or not.
The lack of feedback with shooting enemies detracts a lot, too. It's hard to tell if enemies are being hit or not and hard to tell when your base is under attack.
Also, the lack of variety in turret and enemy types takes away any strategy that could have been there. I basically just loaded up turrets in between the spawn points and my base, and got rich quick. :P
I think if you add some more complexity and include some feedback for the player, it could be a very fun TD game.