When Constantine I defeated Licinius at the Battle of Chrysopolis in September 324, he effectively became the emperor of the whole of the Roman Empire. Just two months later, Constantine laid out the plans for a new, Christian city to replace Byzantium. Intended to replace Nicomedia as the eastern capital of the empire, the city was named Nea Roma (New Rome); however, most simply called it Constantinople ("the city of Constantine"), a name that persisted into the 20th century.Six years later, on 11 May 330, Constantinople was proclaimed the capital of an empire that eventually became known as the Byzantine Empire or Eastern Roman Empire
The establishment of Constantinople served as one of Constantine's most lasting accomplishments, shifting Roman power eastward and becoming a center of Greek culture and Christianity.Numerous churches were built across the city, including the Hagia Sofia, which remained the world's largest cathedral for a thousand years. The Ecumenical Patriarchate of Constantinople developed in the city, and its leader is still one of the foremost figures in the Greek Orthodox Church. Constantinople's location also ensured its existence would stand the test of time; for many centuries, its walls and seafront protected Europe against invaders from the east as well as from the advance of Islam.During most of the Middle Ages and the latter part of the Byzantine period, Constantinople was the largest and wealthiest city on the European continent, and during parts of this period the largest in the world