Thanks for playing it, and the good feedback! I'll update the description to make it more explicit.
The cost of the towers was a deliberate thing to try to balance the game. The idea I had for the jam initially was that the power grid would only be able to hold so many towers, so there would be relays like boosters or switches that allowed the power to be diverted. The increasing cost was my compromise in the time frame so that there would actually be a challenge. But even then I worried there would be no real challenge because the tower placement is all by necessity. "What can I turn on at this moment that will help with this wave?"
It maybe was a bit too brutal for a jam though. But with one day to go I was despairing because I had all the pieces in place but there was no player agency at all. Either the player got overwhelmed or built up an overpowered defence and there was no challenge.