Excerpts from Books and Wikipedia
"The Byzantine Empire, sometimes referred to as the Eastern Roman Empire, was the continuation of the Roman Empire in the East during Late Antiquity and the Middle Ages, when its capital city was Constantinople (modern-day Istanbul, originally founded as Byzantium). It survived the fragmentation and fall of the Western Roman Empire in the 5th century AD and continued to exist for an additional thousand years until it fell to the Ottoman Turks in 1453. During most of its existence, the empire was the most powerful economic, cultural, and military force in Europe." [Wikipedia]
Constantinople had more wealth and population than Rome. Until falling to the Ottoman Turks in 1453, the Byzantine emperors were the center of the Greek-speaking world. [Furtado: 1001 Days]