Nice prototype!
Definitely too easy right now, but also too slow I'd say. The upgrades didn't feel meaningful enough - I think if your power increase was way more obvious but the game was also way harder it'd feel better.
I also think you could theme the currency as power accumulation that you're sending to factories to produce the turrets, maybe even keep it as the win resource so there is some risk vs reward!
Love the art style and atmosphere, I think it has potential :)