The problem with factions in a game that covers such a vast period of time (come on, 500 years for one part is a rather long time) is whether or not to go with states or nationalities- for example, the Greeks could be considered a single nationality or culture, but between 500 BC and 0AD there were many states within that nation, the Athenians, the Spartans, the Thebans, Corinthians, Epirotes, Macedonians ect. ect., which rose and fell within that period. Or it could go the other way where within one state there were many nations- within the Persian Achaemenid Empire there were Perisans, Medes, Arabians, Turks, Thracians, even some Indians and Greeks. 0ad seems to have gone for a combination of both.