The Citadel is a lone city based on a located on a barren, desolate planet with a deadly atmosphere mostly composed of methane located in the Forgotten Zone.
It is inhabited and protected by a race of Goblin-like aliens. They enter and exit from a checkpoint run by two of the Goblins. When they return from periodic missions, the vehicles enter the checkpoint and wait in a room until it is vented of Methane before proceeding into the city. Hal Jordan, Kilowog, and Razer infiltrated the city by sneaking aboard one of the Goblins' envoys. They were soon captured and forced to fight Mother in an arena. Once Jordan and Kilowog recharged their rings off a Manhunter skull, they defeated Mother and left the city with Razer.