I download it in 8/1/2019 and at that time I play a start I think, but recently I replay it completely and think it's absolutely amazing! and that unexpected ending is very pleased and all worth for everything player did.
Then I come and read log, comment, etc, then I found this https://itch.io/t/80351/feedback you said the problem with tower cost.
When I see it I know there's a simple solution which is used by other games, that is, increasing cost with tower numbers, e.g. first tower cost 100, second cost 200...
I also read in other comment somewhere, you said the game system should be simple, that is what makes it so best, I absolutely agree so, and I think a simple way is give game more modes, like classic simple system mode, adv system mode. or even, different game archives versions if you want keep a simple menu?
For right now, I hope the game can has a little change: HP of emeny ship can be stronger and stronger with game process, otherwise the emeny is weak when you are in late game. This function can be an option in menu, and more, give player access to set max value.
Anyway, amzing game! art, simple, city building, and more good things, I had completely played it 3 or 4 times recently.