This is a lot cleverer than I was expecting, you really made it feel like a tower defence, and I definitely made use of some of the same strategies. 

Upgrades work well, though I wish there were more, and maybe specializations? (After taking the ice upgrade, you can't take fire, etc.) servicable for what you have here though. 

I quit after beating three waves without having to move, so needs some balancing. Even just the occasional boss mob would be welcome.