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As a symbol and expression of the universal prestige of the Patriarchate of Constantinople, Justinian built the Church of the Holy Wisdom of God, Hagia Sophia, which was completed in the short period of four and a half years (532–537).
Things went worse for Byzantium during the civil wars that followed after Andronikos III died. A six-year long civil war devastated the empire, and an earthquake at Gallipoli in 1354 devastated the fort, allowing the Ottomans (who were hired as mercenaries during the civil war by John VI Kantakouzenos) to establish themselves in Europe. By the time the Byzantine civil wars had ended, the Ottomans had defeated the Serbians and subjugated them as vassals. Following the Battle of Kosovo, much of the Balkans became dominated by the Ottomans.
The Emperors appealed to the west for help, but the Pope would only consider sending aid in return for a reunion of the Eastern Orthodox Church with the See of Rome. Church unity was considered, and occasionally accomplished by imperial decree, but the Orthodox citizenry and clergy intensely resented the authority of Rome and the Latin Rite. Some western troops arrived to bolster the Christian defence of Constantinople, but most Western rulers, distracted by their own affairs, did nothing as the Ottomans picked apart the remaining Byzantine territories.
Constantinople by this stage was underpopulated and dilapidated. The population of the city had collapsed so severely that it was now little more than a cluster of villages separated by fields. On 2 April 1453, Sultan Mehmed's army of some 80,000 men and large numbers of irregulars laid siege to the city. Despite a desperate last-ditch defense of the city by the massively outnumbered Christian forces (c. 7,000 men, 2,000 of whom were foreign), Constantinople
finally fell to the Ottomans after a two-month siege on 29 May 1453. The last Byzantine emperor, Constantine XI Palaiologos, was last seen casting off his imperial regalia and throwing himself into hand-to-hand combat after the walls of the city were taken.
Aftermath
Mehmed II went on to conquer the Greek statelets of Mistra in 1460 and Trebizond in 1461. The nephew of the last Emperor, Constantine XI, Andreas Palaeologos had inherited the defunct title of Byzantine Emperor and used it from 1465 until his death in 1503. By the end of the fifteenth century, the Ottoman Empire had established its firm rule over Asia Minor and parts of the Balkan peninsula. Mehmed II and his successors continued to consider themselves proper heirs to the Byzantine Empire until the