The citadel is one of FunFrock's most important prisons, which gave its name to the island where is located: Citadel Island. Part of the building's interior is very similar to the one of FunFrock's Secret Fortress.
Twinsen's first adventure begins here. After seeing strange dreams (later revealed to be messages from Sendell), he is arrested by FunFrock, being the Heir who will destroy him. In his cell, he wakes from another such dream and is approached by a nurse on a hovering elevator, who tells him to be quiet. Twinsen takes the opportunity and kills the nurse to get out of his cell, which leads him to fight a couple more of nurses and doctors. He can enter a room where he can retrieve the ID card and the holomap that were taken from him when he got imprisoned.
Once outside the citadel, Soldiers suspect Twinsen's outfit and attack him. He can't get through the gate, which only opens for authorised people or vehicles, so to successfully escape the citadel, Twinsen has to hide in the trash in a discreet behaviour, and board the garbage truck.