Constantinople (Istanbul), 1500 years old
Constantine the Great established the new city of Constantinople in 330 AD, creating a Greek-speaking Christian empire that lasted 1,100 years. The Eastern Roman Empire already existed, but Constantine anchored it by establishing his capital at the old city of Byzantium, hence the Byzantine Empire (and endless confusion for history novices).
Mohammed founded the religion of Islam, which was really a monotheistic reform of Bedouin polytheism. The self-appointed prophet died in 632 AD. A few years later, Muslims took Jerusalem from the Christian Byzantine Empire. The Crusades, starting 1095 AD, took back Jerusalem and protected Christian pilgrims who were being captured and sold into slavery by the Muslims.
Islam continued to expand toward Europe. The Ottoman Empire, starting in 1299 AD and eroding into modern Turkey in 1923, was an enormously rich military empire stretching out toward Europe. Muslims had to get past Constantinople to enter Europe.
Using siege artillery and overwhelming numbers, the Ottoman Turks finally captured the great and ancient city, turning Hagia Sophia into a Muslim mosque. The event was so traumatic that Eastern Orthodox members still mourn the loss of their city and church on this day, May 29th, each year.
Constantinople stood between the Ottomans and Europe before 1453. The Muslims began a steady campaign toward Europe, standing at the gates of Vienna more than once. The Muslim armies so threatened the Catholic emperor, Charles V, that he left off persecuting the Lutherans from 1530 (when he heard the Augsburg Confession) until the death of Luther in 1546. The Muslim threat gave Lutherans a chance to become established before Charles V turned his attention back toward Germany.