It is definitely best to use the purple towers (upgrading them does not seem to really help). Also you should definitely put your money in the bank for first wave and withdraw it by the time you have collected enough interest to get your first purple tower; then withdraw the money buy the purple tower and defeat the first two waves without losing 1 life (that is if you time it right).
I have beaten the game all 50 levels without losing 1 life. Upgrading the purple towers does not seem to help, or buying any other tower.
There are particular wave types I find difficult to beat. If you put your money into bank click before first wave click through all levels you earn slightly more than 1000 gold, so if you want to make a lot of gold in this game you need to kill a lot of enemies.