Thank you for trying my game! As far as the fireballs are concerned. They are indeed hitting the enemies. My problem is that I didn't give enough player feedback to let you guys know that they are hitting. The main problem is the collider on the fireball is too large so it's hitting the enemies without making a visual connection on screen. Someone also suggested that I add in a particle system for when a fireball hits and I think that would be a great idea as well.
I can totally get where you're coming from with the music. I wanted to do a lot more with the audio in the game with providing a sounds option to turn down the SFX and music volumes if players wanted. But it was running down to the end, so I added in sounds the quick and dirty way just trying to equalize everything the best I could.
I updated the description a bit on how to place defenses. Pretty much you just select it from the selection at the bottom of the screen and then place it on a desired square. There's 4 defenses in all but they're not unlocked right away. The first level has no defenses as I wanted it to be a level for the player to get used to the controls of the player character. Hopefully that helps if you come back to try again! Thanks for the feedback!