Honestly, I have no idea what's happening, but I somehow managed to survive all 15 waves :D But thinking back I'm not so sure now. Hahaha.. Is there supposed to be like a winning scene? I ended up back to the main menu.
As PurpleXYZ said in the comment, the game did get stuck one time for me that requires me to reload the browser. I'm not sure what happened.
It would be nice if there is an introduction or tutorial at the start of the game to tell us a little bit about the game.
Anyways, good job on the game.