The visitor to modern İstanbul actually gets to experience three great cities at the same time. There is Byzantium, established by Greek traders on the site of an ancient city and even older prehistoric settlements. Byzantium became the seat of the Roman emperor Constantine, took his name (Constantinople) and became the greatest city in the Empireand the largest and wealthiest city in the world. Surviving the fall of Rome itself, Constantinople eventually fell to the Ottoman Turks and became İstanbul, capital of the Ottoman Empire and then chief city of modern-day republican Turkey, with great historic sights and buildings at almost every turn.
The area around İstanbul has been settled since about 6000 BCE, and stone age artifacts are today to be seen in museums there. These settlements represent the spread of Neolithic civilization from the Near East into Europe. The area was later occupied by the Phoenecians and was the site of the city of Chalcedon. Greek traders founded Byzantium around 660 BCE and it became a prosperous city and ally of Rome, but little was left after it chose the wrong side in a rebellion against the emperor Septimius Severus.
Byzantium officially became part of the Roman Empire in 73 CE, and 250 years later Constantine decided to build a new Christian city and make it his capital. Although thenceforth Constantinople, the eastern possessions of Rome that it governed became known as the Byzantine Emperor. The Hagia Sophia was built during this period and changed the history of architecture with its huge dome; for centuries the largest cathedral in the world, it was an Orthodox basilica for a thousand years, then a mosque for four centuries and since 1935 has been a museum, which can be visited today. The Hippodrome, built as a race course, was similarly the largest arena in the world for centuries, and parts of it can be seen today in the Sultan Ahmet Square along with the Obelisk of Pharaoh Thutmose, which was brought there from Egypt by the emperor Theodosius. The walls and sea front, some of which still stand, were the defensive perimeter of Christian Europe against Islam until 1453.
The Muslim empire that followed Byzantium was for centuries thereafter the center of Islamic culture. The rich Ottoman history of İstanbul has also been largely preserved in the modern city.