Pretty fun! Only minor changes I would make is maybe a charges system for spawning certain enemy types or a limited rate you can spawn troops at that's pretty low, so you can have a swarm like in the other games of this genre, a UI pop-up when you hover over the attack types, such as how much damage they deal, health, and speed, and some method of gaining money when you are in a bad spot, because if you physically cannot get a object through (particularly in the third level), you have no way to progress. This is great work! These are just nitpicks to be honest, it's great the way it is.