The Romulan capitol was a structure on Romulus.
Commander Tomalak planned to display the broken hull of the USS Enterprise-D in the center of the Romulan capitol as a symbol of the victory of his people. He reasoned that it would inspire Romulan armies for generations to come and serve as a warning to any other traitor, referring to Admiral Alidar Jarok, who would create ripples of disloyalty.
He, however, was never able to bring his plan to fruition as Captain Jean-Luc Picard was prepared and brought reinforcement in form of three Klingon Birds-of-prey with him when entering Romulan space and flying to Nelvana III. (TNG: "The Defector")