How did Constantinople become Istanbul?

NARRATOR: Istanbul today, a junction between Europe and Asia. A city that has endured a great deal. Seven hundred years ago Istanbul was still called Constantinople. The metropolis on the Bosporus was the home of Christian emperors. Colossal walls shielded what was at the time Christianity's largest city. Hagia Sophia - the Church of Holy Wisdom - attests to this illustrious epoch. It was a symbol of the emperor of Constantinople's claim to being the rightful custodian of Christianity. Until completion of St Peter's in Rome 1,000 years later, the Hagia Sophia was the world's largest domed building.

ALEXANDER DEMANDT [translation] "Constantinople was for hundreds of years the cultural summit of Europe. Since the sixth century, one can say until the rise of Paris and London, Constantinople was the most important city of all."

NARRATOR: Very few treasures have stood the test of time, but what has is proof of a refined culture. Still, in the mid 15th century the demise of the capital of Christendom is on the horizon. The successors of Osman I, who reigned over parts of modern Turkey, built a powerful empire in the Orient. Sultan Mehmet is particularly keen to pick the Golden Apple, as the Ottomans called Constantinople. Constantinople's walls have withstood numerous sieges undertaken by Mehmet's ancestors, but Mehmet has an ace up his sleeve: war machinery, the likes of which the world has never seen before.

DR. KLAUS REINHARDT [translation] "They repeatedly fired stone cannonballs at one area using huge cannons. This wore away at the walls and caused them to crumble, making it possible to storm the city through the breaches."

NARRATOR: Constantinople falls into the hands of the Ottomans. The largest city in all of Christendom is now ruled by Islamic powers. The Hagia Sophia is turned into a mosque. Stone tablets praising Allah and the prophet Mohammed are placed there.