Be prepared for mind-screwing solutions to the later levels. They are glorious.
I've played the game during beta, when you still had to pay for upgrades. That was not very well balanced. Now it's fine. Currently awaiting my Steam code, but the puppies can take their time with that, until cloud and achievements are done. ^_^
Also, if you like the faster pace and horde style tower defence more, check out Harvest: Massive Encounter. Only four kinds of defences, but
so much to do.