I beat the marine attack by building 2 archers and a mage. then I juice the mage all the why to lighting master before anything else. I then either build another 2 archer and mage set or bring the original two archers to flame archers first. If you can get to two archmages, a high elf and a barrel elf, you should be ok. the hardest levels are those with the molotov guys. I usually try to have a healer by then. It really helps.
once you realize you have to start slow, not a bad game at all. The badges are hard. Especially since you have to figure out which building level unlocks which dungeon. As well as which you have to complete for the others to open as well.
would have given 5/5 but is so closely resembles desktop TD, with a few changes, all of which I do enjoy. Mega towers rock. Would be nice to be able to see the effects or stats when it is a non damaging Megatower.
Like scyce said go up the middle to the trough, I held the creeper back with just three mortars place where it spills over. this allowed expansion at will. Great concept, I would love to see this with improved graphics and maybe persistent upgrades.