I like the tower defense idea for the theme. Good use of art assets and sound.
In terms of gameplay; I think allowing the player to sell defenders at a discount would be a nice addition as when I put two in the row I spent all my currency and my other sides were unprotected while I wasn't able to get more currency through killing enemies.
The "How to Play" button on the UI doesn't seem to work (I'm using Windows). It would also be nice for this to be a WebGL to allow more players to enjoy this game!