Istanbul a Turkish city located on the Bosporus Strait in the northwestern part of the country. It has historically been known as Byzantium and Constantinople and was a significant city in the Roman and Ottoman empires. Famous structures include Hagia Sophia, built by Justinian the Great originally as church and later became a mosque, and the Sultan Ahmed Bosque, popularly known as the Blue Mosque, built in 1616. Today, Istanbul is Turkey’s most populous city and a hub for economics, history and culture.