Originally Posted by Martok
1.) Historical reasons--a certain romanticism is often associated with the Eastern Roman Empire. One thing to remember is that Byzantium was the dominant Christian power in the early Medieval period. The Emperor's influence was at least as great as the Pope's, if not more. The Empire was also situated in the middle of everything: To the west was Catholic Europe; to the east and north were the pagan barbarians of the steppes; to the south and southeast was the Islamic Caliphate. Byzantium was a place of great intrigue, ruled by men both wise and foolish, both pious and impious. It was a place of enormous wealth, and was also home to some of the holiest places in Christiandom.
2.) Gameplay reasons--in MTW, the Byzantines were a blast to play! Being in the middle of everything, you were faced with many challenges and opportunities. With the Byzantines, there was no one good way to win--you could employ multiple strategies when playing as the Empire. In addition, they had one of the most interesting and varied unit rosters in the game, including the Kataphractoi, Trebizond Archers, and the Varangian Guard.