From the time of King Minos, Hellenic culture has flourished in the Aegean and Eastern Mediterranean. Alexander the Great would spread it across the Known World. The Romans, a non-Hellenic people, would eventually conquer even more then Alexander, including Achea and Alexander's homeland of Macedonia. But they absorbed much of the culture and practices of the Hellenic world, even the worship of the Olympian gods.
In the following centuries Rome ruled, but Greek, not Latin, remained the dominant language of the eastern part of the empire, and the Acheans considered themselves to be culturally superior to the pragmatic, and militaristic, Romans. In 470 BI (using the standard western calendar), Constantine the Great moved the imperial capital to a city then know as Byzantium, and which would become known as Constantinople. He also ended the suppression of a cult that would become the Orthodox and Holy Church. Over the next few hundred years, the western part of the Roman Empire would be destroyed by invasion and civil war, but the eastern part would survive, and exists to this day.
As far as they are concerned, the Eastern Emperor is the Roman Emperor, and the Patriarch of Constantinople the spiritual leader of the Orthodox faith. They are not comfortable with a Germanic king calling himself Emperor, or the Pope thinking that he is more then another Patriarch. Much worse though is the Ottoman Sultan to the east. The Ottomans seem to spread their empire as easily as the early Caesars did, and have to be appeased with regular tribute, or held off with the help of western crusaders. Neither option is a palatable one for the Byzantines, but at least the ancient empire survives.
The Eastern Empire contains a number of distinct regions. The Roman Republic and Achea are largely autonomous, and seek to keep things, as they were many centuries before. The worship of the Olympians, banned in the rest of the empire, flourishes in these regions. Much of the rest of the empire is populated by Slavs who came first as conquerors, then where conquered and assimilated in turn. Bulgaria is the largest Slavic region. The Slavs are Orthodox, as are the Byzantine Greeks that make up the heart of the empire.
Ancient academies of wizardry are maintained in Rome, Constantinople, Athens and some of the other Achean cities. Incredible knowledge can be found in these places, built up over many centuries. Savants from the west are occasionally allowed access to their libraries, and are always amazed at what they find.
There are other kinds of knowledge as well and other kinds of magic. Philosophers still teach as they have for millennia, and a few are taught in turn by the Centaurs, just as the heroes of ancient myth where. While elves and dwarves are rare, giants are said to walk in distant mountain passes, and in isolated glades a satyr's pipes may still be heard, a dryad glimpsed. Things change slowly in the Eastern Empire.