I love that you have a README and License file (main criteria for open source category). Rather comprehensive and well structured repo, nice work!
Loving the sound effects! Music is great and I love that it gets louder when you start the wave. Graphics are artwork fit nicely with your game. I'm really enjoying the death animation... fireworks!
Super addictive, loving the use of an in-game shop, and the different balance bars are cool. Really good job and great work on a polished game. You've put a lot of work into this and it shows, kudos.