Istanbul is the world's only city on two continents: A narrow sea strait, called the Bosphorus, divides the city in two, with the western half of it in Europe and the eastern half in Asia. Two long suspension bridges, namely the Bosphorus Bridge (1973) and the Fatih Sultan Mehmet Bridge (1988) link the two parts of the city, as well as connecting the two continents. In its long history, Istanbul (Constantinople) has served as the capital city of the Roman Empire (330-395) , the Byzantine Empire (395-1204 and 1261-1453), the Latin Empire (1204-1261) and the Ottoman Empire (1453-1922). In 1592 consecutive years between 330 and 1922, a total of 126 emperors and empresses have ruled over four vast empires from the shores of this immortal city; where world-famous historic buildings, such as the Hippodrome of Constantinople, the Haghia Sophia and the Topkapi Palace stand next to modern skyscrapers.
Istanbul has constantly been inhabited since the Paleolithic and Chalcolithic periods. The oldest known human settlement, the Fikirtepe mound on the Anatolian side, has artifacts dating from 5500-3500 BC. In nearby Kadiköy (Chalcedon) a large Phoenician port settlement has been discovered. Chalcedon was also the first location which the Greek settlers from Megara chose to colonize, in 685 BC, a few years before they colonized Byzantion on the other (European) side of the Bosphorus, under the command of King Byzas, in 667 BC. Byzantion itself was established on the site of an ancient port town named Lygos, which was founded by Thracian tribes between the 13th and 11th centuries BC along with the neighbouring town of Semistra. During the Roman period Byzantion was renamed as "Byzantium", and Roman emperor Septimius Severus temporarily made Byzantium his capital city in the late 2nd Century AD, renaming it as "Augusta Antonina" in honor of his son. But the man who gave this city its real importance is, without doubt, Constantine the Great, who renamed it as "Nova Roma" (New Rome) and officially made it the new capital city of the Roman Empire in 330 AD. After Constantine's death, the name Nova Roma was changed to "Constantinopolis". Even though Constantine was the first Christian emperor, it was Theodosius the Great who made Christianity the official state religion by a law in 381 AD. Thus, Constantinople (Istanbul) also became the first Christian capital city of the Roman Empire. In 395 AD, after the death of Theodosius the Great, the Roman Empire was divided in two parts between his sons; with Rome as the capital city of the West, and Constantinople as the capital city of the East. For more than 1000 years between 395 and 1453, Constantinople served as the capital city of what was called the "Medieval Roman Empire", later renamed by historians as the "Byzantine Empire". As a living legacy of its Roman past, Constantinople (Istanbul) is still the seat of the Ecumenical Patriarchate, the highest authority of Eastern Orthodox Christianity. On May 29, 1453, Ottoman Turks under the command of Mehmed II "The Conqueror" defeated the army of Constantine XI Palaeologus and made Constantinople their new capital city. Between 1453 and 1922 Constantinople served as the capital city of the Ottoman Empire, also being the final seat of the Caliphate (the highest authority of Islam) between 1517 and 1924; since Ottoman emperors were the last rulers carrying the title of "Caliph". In 1923, with the establishment of the Turkish Republic, Ankara became the country's new capital city, and the