Cardassian Union insignia image.
The Citadel, also known as the Republican Citadel, was a structure located on the planet Cardassia. It was situated in the capital of the Cardassian Union and contained the offices that belonged to the Cardassian Gul-tar. (DS9 novel: Objective: Bajor) The building actually dated to the times of the Republic and was partly constructed from the ancient volcanic rock the Hebitians used to build their structures with during ancient times. (DS9 short story: "The Calling")
In 2371, Gul-tar Keve held his spartan offices in the Citadel which oversaw the rest of the city. Gul Skrain Dukat met with Keve in these chambers where he was told that a rebel movement was active in the Cardassian government which sought to overthrow the military. (DS9 novel: Objective: Bajor)
After the Dominion War, in 2380, Cronal Gys asked Elim Garak to meet him on the grounds adjacent to the Citadel in order to discuss the fate of Kel. (DS9 short story: "The Calling")