Overgrown Battlement and Axebane Guardian, along with the 15 other defenders, reel in the mana to cast the fat creatures. Carnivorous Plant and Grave Bramble add some prevention with their power. As far as mana ramping goes, respectfully, I think the Overgrown Battlement and Axebane Guardian combo is far better than the similar Priest of Titania and Elvish Archdruid combo goes, because the defenders usually offer a better trade off. They are both harder to kill, and better at defense, such that one can have some time to bring out the beat sticks better than Elves do.
The defenders, or "Walls," is how this deck got its name. It is a spoof off of the Christmas song, "Deck the Halls."