I don't mind simple graphics, I love playing old games.
The money is out of balance, you are getting too little for the costs of the new towers, at lvl 49 you get 80$ you can't buy 1000$ upgrades for it.
But the money fits the waves tho, altho maybe even a bit too easy. You have to fit them all together too make an exciting tower defense game.
It's also quite slow-paced. Which gets rather dull after a while.