In the approximately 1000 years of the existence of the Empire, Constantinople had been besieged many times; it had been captured only once, during the Fourth Crusade in 1204. The crusaders, however, had not originally set out to conquer the Empire, and the Byzantines re-established themselves in the city in 1261. In the following two centuries, the much-weakened empire was gradually taken piece by piece by a new threat, the Ottoman Empire. In 1453 the "empire" consisted of little more than the city of Constantinople itself and a portion of the Peloponnese (centered on the fortress of Mystras); the Empire of Trebizond, a completely independent successor state formed in the aftermath of the Fourth Crusade also survived on the coast of the Black Sea.