Great submission, this has lot's of potential!
Some changes that i would love to see:
- make the energy system obvious, it took 3 tries for me to understand why things weren't working
- quick button for turning buildings on and off (or even a customizable preset that will switch buildings for you in the battle phase)
- unit variety and an indicator that will show where the enemies will come from (this is a personal preference)
- bug: if you restart you lose the upgrade button?
I can definitely see this game sell well on steam for a couple bucks after some polishing and more content!
Good job :)
P.S.: Were you inspired at all by 9 kings?