Shimrra's Citadel was the palace of Shimrra Jamaane, the Supreme Overlord of the Yuuzhan Vong, after his arrival on Coruscant, which had been renamed Yuuzhan'tar. It was based in the Sacred Precinct, close to the remains of the Imperial Palace.
Originally a Koros-Strohna, the Citadel was capable of returning to space if the need arose. It had a spire of yorik coral on the top, giving it the appearance of a mountain, and flew "up right" in space. Dovin basals manipulated the area around the Citadel to give the appearance that rainbows were emanating from it.
There were separate entrances for the four ruling castes, all of which ended at the Hall of Confluence, Shimrra's public throne room, where Shimrra would hold court with the Elite. Elsewhere in the Citadel, the Warmaster's command grotto was grown for the Warmaster of the Yuuzhan Vong, so that he could prepare for an assault.
At the top of the spire were Shimrra's private quarters, his coffer, and a yorik-trema that served as his personal escape pod. The private throne room had a moat of black Yuuzhan Vong blood between the entrance and Shimrra's throne. In both the Hall of Confluence and the coffer, there were dovin basals that caused the gravity to increase as they approached the hau polyp throne, making those who approached the Supreme Overlord feel like they were walking uphill. The Shamed One Onimi, Shimrra's jester who was the true Supreme Overlord, was capable of manipulating the Citadel independently of a yammosk, dhuryam, or even Shimrra's own control.
During the Battle of Yuuzhan'tar, the Citadel was breached by the Galactic Alliance troops, including Page's Commandos and YVH 1 droids, thus defiling the structure. The Citadel was linked to Onimi, so it self-destructed after he died.