Byzantium Greek city, capital of Thrace, in the western part of the entrance to the Bosphorus Strait, a part of what is today Istanbul, and which has been important in history since its foundation. In the Middle Ages, the Byzantine mperium was made up of all the eastern territories that belonged to the Roman Empire. It was officially created in the year 395, when the western and eastern territories were definitively separated. Its rise and fall mark the beginning and end of the Medieval Era