Nice music, art style and a solid implementation, the game only start to loose some fps when the screen is completely filled with enemies.
I only wish there were more decision making, I've always spend my money right away and the monsters never reached the tower. Maybe some durability so the player would have to keep an eye and change the balistas from time to time or another type of weapon that is better against a certain monster.
But at this point i'm only nitpickying, it's a solid entry anyway :)