BYZANTIUM
Byzantium was the original name of the city that the Roman Emperor Constantine the Great made his eastern capital and renamed Constantinople. When the western empire was later overrun by barbarians, Rome ceased to function as a capital and Constantinople became the capital of the Empire. Known as the second Rome, it remained the capital of the Byzantine Empire (successor to the Roman Empire in the east) until 1453, when it was conquered by the Ottoman Turks. (Another point that has been used to mark the end of the medieval period.)
The Crusades
During the Crusades (11th-13th Centuries), there were many disagreements and sometimes fighting between the westerners and the eastern emperors; at one stage the western lords imposed an unpopular "Latin Kingdom of Constantinople" on Byzantium, but this did not last long and the Greek Emperors returned in 1261. However, Byzantium was, through all those years, the guardian of the back door into Europe. If Byzantium had fallen, western civilisation as a whole would quite likely have fallen too.
Miklagard
This was the Norse name for Byzantium, and is frequently used in novels where the Emperor's celebrated Viking Guard (the "Varangians") make an appearance.
Histories
The best are John Julius Norwich's Byzantium: The Early Centuries, which takes up to Christmas Day 800, Byzantium: The Apogee, which continues the story to the coronation of Alex Comnenus in 1081, and Byzantium: The Decline and Fall, which takes us up to the final conquest of Byzantium by the Ottoman Turks in 1453. Also excellent on the early years of Byzantium is Peter Brown's The World of Late Antiquity.
