The Ziggurats are 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. They can be upgraded to Spirit Towers or Nerubian Towers. The only visible difference between the three types is the crystal above them.