For level 12 start your build like this:
One you get some more money add additional towers like this (sorry I can’t show all towers at once):
Note: You must build the Totems last, and you must press shift while placing all 4! Otherwise you will only be able to place 3 of them.
Also: The start is a bit hard, you will need some spells. But once things get going it’s trivially easy, so don’t worry about using up your mana at the start.
Buld the 3rd tower as soon as you have cash for it. Then upgrade the camp, then upgrade the 3 towers. Once you’ve done the second to last upgrade (i.e. not venom or frost), start building and upgrading the last 2 towers.