The upgrade panel for the individual tower that shows up at the bottom of the screen seems to go away when trying to click the tower then click upgrade, it's pretty hard to reach it and takes quite a few tries. If the game is updated after the jam that may be a place to start improving it.
Other than that I liked the simple clicking as well as the different towers and options in the shop. I found the exchange rate timer and truck really interesting and it added to the gameplay!