The town of Byzantium, chosen as the new capital of the Roman Empire by Emperor Constantine, continued to develop as Constantinople, the capital of the Eastern Roman or Byzantine Empire. In addition to its strategic location, the city attracted attention worldwide due to its fascinating monuments, including the Church of Hagia Sophia. Constantinople resisted several sieges by the Muslim Arabs, but suffered greatly during the Fourth Crusade. After centuries of resistance to invasion, the city was finally conquered by Sultan Mehmed II in 857 / 1453. When the Ottomans conquered the Mamluks in 923 / 1517 and took the title of caliph, their capital became the centre of the Islamic world and was known by the names: Konstantiniyye, Asitane, Dersaadet and Istanbul. Istanbul was the Ottoman capital until 1922; for 400 years each sultan had adorned the city with new monuments and updated the infrastructure.