[quote=@Kho]
Hmm, I'm slightly confused. Does each character control an entire nation, along with provincial tribes/officials/noble families, or is there a different player for each region?
[/quote]

With possibly the exception of the Caliphate - where exceptions can be drawn - Rome and Persia are larger entities made up of smaller player-controlled entities playing politics under the guise of that Empire. For Rome, it's the diocese. And in the Persian Empire it's the smaller noble families landed across the Empire.

Alternatively below that are smaller groups like volunteer armies or mercenary forces from outside the mapped region moving in to get involved with the region at large.