The gameplay is nice, although jumping doesn't seem to serve any purpose. There's some issues though: you can buy spawn times to under 0 into negative numbers, which just wastes money and doesn't increase the spawn rate; the dying sounds of monsters and soldiers becomes really loud and annoying when a lot of them die at the same time, and when having really high spawn rates the game slows down A LOT because of so many things on screen.
Maybe you could add upgrades to the hero's walking and/or attack speed to help with the monsters piling up too much