I had the same thought about the background sound - Head cannon was I was playing this table top tower defense game in Ye Olde Game Parlour, especially because the music was so chill. Something a little more martial sounding would change the feel significantly.
I was starting to think that my armaments would always out scale the enemies. I finally met my match though. I couldn't quite tell, but it also looked like placement almost didn't matter. Tower's ranges were incredibly far. Some variation could make things more interesting.
All this constructive criticism aside. This is amazing for 10 days of work! Something to be very proud of. The effects of the towers firing were really cool. The different cards and the economy. Plus you had time to put in a json writing level maker! Very cool! You could do a lot with this tower defense game.