Good job! I think with the way the routes work there are some slots which are strictly better than others - so maybe more could be done to even out the strength of different positions - maybe height, or certain turret types could only go in certain positions, etc.
I think you could also add a bigger variety of turret types - maybe you have to choose the type of turret to upgrade to - one is faster and one is stronger - or one has a different type of attack - maybe one that slows down enemies or traps them in some way - or one that strengthens turrets near them (a reason to not socially distance maybe? to force that trade off).
Great effort for the game jam though :) I hope you expand on it!