Figured I should start responding to some of these comments lol
I'm glad you enjoyed! The really fast planet progression has indeed been a common complaint, and replaying the game I totally get it. I think there are two culprits here.
- Once you get a tower setup you really like, there isn't any reason to gamble for more towers outside of experimentation. We initially planned to add tower upgrades that would buff damage, fire rate, functionality, etc. In fact, there's functionality for increasing the level of a tower; you would take a tower from the slot machine and add it to the same tower in an existing slot. However, I didn't have time to code what the levels actually did for each tower or the UI showing you those functions, so it had to be scrapped.
- I set the planet upgrade cost way too low. This is because I hadn't actually playtested the game through to wave 15 until after I'd submitted to itch. I should've fixed it during the editing period, but I was preoccupied with getting audio in and fixing bugs.
All this is to say that your idea of upgrading towers is good and something we wanted to add! We just didn't have time :(. Glad you found the game fun regardless of the squished progression