At first I thought that towers removed at the end of the round were not refunded, which would have been a very cool mechanic to increase the difficulty. As a result, I tried to beat each round with as few towers as possible. I later realized that they are refunded.
Currently there's a glitch(?) where multiple towers can be built on a single square, so you can build infinite towers if you want.
I'm not sure when (if ever) the fossil fuels begin outscaling the player money+towers. Around round 30, I can afford 30x as many towers as I need to beat the level.
This is a nice intuitive simple game which could be more fun if it were made a lot harder.