The game needs a lot of balancing. I would recommend to reduce the number of upgrades, rethink which make sense to keep.
The lightning is super overpowered, you can wipe out many levels thanks to it... until you come against hellfire. Which I don't know how I am supposed to beat since I had all upgrades and full army... (also lots of money, but nothing to spend it on) - and it reliably killed everyone in two hits. That's not fun, it's first too easy and then impossible.