Castle Ostia oversees the city of Ostia, which in turn is the capital of the Lycian territory of Ostia. Ostia is the head territory of the country of Lycia, as the Marquess of Ostia is always head of the Lycian League.
Castle Ostia is a mighty fortress, famed for being so well built and tactically designed that no enemy could breach its walls. The castle is defended with reinforcing the halls and openings with Ostia's famed armor knights and generals.
Though claimed to be unconquerable, the castle itself was breached only twice in Lycia's history. The first of which occurred 980 years after The Scouring, when Nergal used teleported his morphs in in an effort to lure Lyn, Hector, and Eliwood into returning to the Dragon's Gate. Since Nergal merely teleported his morphs inside, he did not technically breach the walls, but only bypassed them.
The second and greatest of the castle's defenses occurred years later, during Bern's assault on Lycia. Bern, with the help of rebels against the nobility, subdued the mighty Hector, and conquered Castle Ostia. Roy, however, counter assaulted with his army of companions and through the help of Lilina, Hector's daughter, the Castle was Lycia's once again.