This is neat! This is very addictive, and it has a lot of potential for the future. Of course, sound, but beside that I loved it. You could add more variety in the towers and upgrades. The ultimate thing to do would be to add a deck building mechanic. Like before starting the game you choose which cards you want to use and then during  the game the draws are from that deck. This way you could go crazier with the towers mechanics, and it would create a HUGE replay value. (+ maybe some booster openings? with currency you gain depending on your score, it would be a neat progression system). So yeah, please continue this :)