Byzantium is a playable civilization.
The Byzantine Empire, centered around the city of Constantinople, was the successor to the Roman Empire. At its height, the Byzantine Empire reached from Persia to Spain, controlling nearly all the territory that felt the lap of the Mediterranean's warm waters.
When the Byzantine Empire began and the Roman Empire ceased is a point of much contention. Some historians place the beginning of the Byzantine Empire in 300 AD, when Constantine I moved the capital of the Roman Empire from Rome to a city called Byzantium (later know as Constantinople). Others place the birth of the Byzantine Empire at the end of the reign of Theodosius I, who at his death split the Roman Empire into the Western Empire, ruled from Rome, and the Eastern Empire, ruled from Constantinople.
Regardless of historic debate, after the sacking of Rome in 476 AD, the Byzantine Empire was to become the foremost European nation in art, trade and stability - although not necessarily all at once. Taking the throne in 527 AD, Justinian I extended the grip of the Byzantine Empire from Constantinople to the furthest coasts of the Mediterranean Sea. However the Empire would not be able to enjoy the benefits of such territorial expansions for long, as Justinian's successor, Justin II, would begin a trend of placating the various enemies of the Empire - the Sassanid Persians, the Bulgars, and the Arabs, newly organized under Mohammed - with territory in exchange for peace.
Territorial losses would continue to weaken the Byzantines until a new lineage of emperors, known as the Macedonian Emperors, arose to invigorate the Empire once again. Basil I founded the Macedonian line in 869. Under the Macedonians, the Byzantine Empire once again began to reach outwards, solidifying its hold on much of the territory neighboring Constantinople. Art flourished in their roughly two centuries of rule, as did the Greek Orthodox Church.
The new millennium would be a painful one for Byzantium. Relations between the Roman Catholic Church and the Eastern Orthodox Church had been deteriorating for many years, and the conflict came to a head with the Great Schism (1054), which finally split the two religions. Militarily, increased assaults by the Seljuk Turks culminated in a resounding defeat for the Empire at the Battle of Manzikert, the Empire losing much of its freshly-gained holdings.
But once again, the rise of a powerful new dynasty reversed the Empire's decline. This new dynasty, the Comnenius, sent the call to Western Europe for aid in fighting the Seljuk Turks. Pope Urban II, seeing a chance to reconquer the Holy Lands of Antioch and Jerusalem, as well as halt the squabbling of the European Kings, spread the word for the First Crusade.
Through the Crusaders' efforts, the Byzantine Empire regained a portion of its lost territory. But the holy warriors of Western Europe became difficult to control, often threatening to assault the very Empire that had requested their aid. John II and Manuel I, the two most prestigious of the Comnenius emperors, held off the Crusaders and safeguarded the empire from a host of external foes, while ushering in a new era of Byzantine artistic and economic prowess.
Manuel I Comnenius was the last great emperor of the Byzantines and the last Comneni. The Angeloi dynasty, successors to the Comneni, proved unable to halt the Empire's slow collapse. In 1204, the knights of the Fourth Crusade, summoned by a disposed Angeloi prince, sacked Constantinople and split the Byzantine Empire into three smaller kingdoms. Nicaea, one of these ex-Byzantine states, recaptured Constantinople years later, but all hopes of reviving the Byzantine Empire would be ended with the capture of the city by the Ottoman leader Mehmed II in 1461.