These towers of the ancient spider kingdom of Azjol-Nerub are called into service once again to provide a source of raw energy for undead forces. Several ziggurats are required to sustain a high number of Scourge warriors. In time, these towers can be further empowered to provide protection.[1] They can be upgraded to Spirit Towers or Nerubian Towers. The only visible difference between the three types is the crystal above them.