oh and maybe add a time slowdown when you remove a tower? (because the enemies at later waves just go at mach speed lol)
also you could make so that the first two towers always get placed with as many upgrades as possible (you won't really lose the money, and its not like you will use it anywhere else)
and perhaps make the freezing tower a fixed cost but its always fully upgraded? I say this because that tower is only really needed at later waves, and by that point you will need that tower fully upgraded anyway so you can slowdown all the enemies.
But anyway those are just some quality of life changes you could add later if you want..