Not the best, but decent
Being able to make your own path makes the game too easy. (make the paths go from one point to another and let the paths overlap) this makes it so you can utilize the towers/units to the fullest.
Also, upgrades arn't enough, you could for instance for one unit, instead of flamthrower have it start out as a infantry with a SMG. then once you fully upgrade it let it reclass into 2 diffent units, say infantry with flamethrower, infantry with assault rifle.
Also instead presetting the plots in the begining, make it so it's clear, and then with each click it places a plot. this makes it easier to set up the maze
Its just an idea