The Byzantine Empire
The Byzantine Empire began when the Roman Empire split and Byzantine became the eastern half of the Roman Empire.
Byzantine's capital was Constantinople. Their religion was Greek (eastern) orthodox, and they spoke Greek. The Byzantine contributed art,architecture, Justinian's code and it preserved greek and Roman culture for today's society.
The Byzantine lasted so long because it had an organized government, it did not get too big, and the people minded their own business.
During the mid 6th century, the Plague of Justinian wiped out roughly a third of the empire's population, creating major military and financial difficulties.