"The Power of the Emperor"
While in control of Constantinople, there could be no civil war and the cities in the core (Greece) cannot declare independence.
Briefly, this reflects the historical ebb-and-flow of Byzantium's fortunes; decline followed by resurgence under a strong dynasty and ensures that Byzantium will remain alive for an appropriate length of time.
Full discussion here: http://forums.civfanatics.com/showthread.php?t=250322&page=17#333
BYZANTINE start: 500 AD
Leaders: JUSTINIAN I
Favoured Civic: BUREAUCRACY
Favored Religion: ORTHODOX CHRISTIANITY
Unique Unit: CATAPHRACT (Early heavy cavalry)
Unique Building: HIPPODROME (replaces Ampitheatre)
Unique Power: POWER OF THE EMPEROR (see above)
Justinian (Great General)
Heraclius (Great General)
Basil II (Great General)
Constantine (Great Engineer)
Anthemius of Tralles (Great Engineer)
Unique Historical Victory:
1. Constantinople is the largest and most cultural city in 1000 AD.
2. Control Constantinople, Greece and Anatolia in 1300AD
3. Be the richest civilization in Europe in 1500AD
Files for Code
Here is version 2 of an encoded settler map, telling the AI where to settle: