Istanbul (Turkish: İstanbul) is the largest city in Turkey, and the second largest city in Europe after Moscow. It straddles the Bosphorus between Europe and Asia; it is the only city to be located on two continents. The urban area has a population of 13 million. Istanbul is the 9th most visited city in the world, containing landmarks like the Hagia Sophia, Grand Bazaar and the Blue Mosque.
Istanbul was founded as Byzantion by Greek colonists in the 7th century BC. It was a small town until Constantine the Great established it as the capital of the Eastern Roman Empire, and renamed it Constantinople. It became the largest city in Europe, with hundreds of thousands of inhabitants. Massive fortifications were constructed by later emperors to protect it from invasion. Justinian the Great constructed the Hagia Sophia in the 6th century AD.
Constantinople was besieged unsuccessfully numerous times by various forces, including Vikings, Arabs, and finally the Ottomans, who succeeded in taking it in 1453 with the help of the largest cannon constructed to date. The Ottomans moved their capital there, converted the Hagia Sophia to a mosque, and built Topkapi Palace.
After the fall of the Ottomans and the establishment of a new Turkish nation in the 1920s, the capital moved from Istanbul to Ankara in the central part of the country. Istanbul has continued to grow, and overpopulation and pollution have become major problems.