DTD is in the third set of Special Ops maps that you unlock. you unlock it by finishing all five of the Frigg missions in conquest. the map is based off a popular tower defence game called Desktop Tower Defence (DTD) made by Paul Preece. The creeper comes from where the enemies in the game would come from and your base is where enemies in the game are usually headed. There are elevations in the ground shaped like towers in the game and many survival pods and upgrades sitting on them.
Strategy Step by Step: