Also, some faction's troop recruiting areas are larger than others, Carthage's AoR is pretty small, reflecting their reliance on mercenaries, the Celtic roster spreads all over Europe. Rome, Hayasdan, and Parthia's AoR expands with reforms. It's been a while since I've played a nomadic faction, but I recall being able to recruit any unit anywhere on the map. Even with factions that have a massive AoR, you will still probably only be able to recruit the best units in your homelands, or in specfic zones(IE:Spartans in Sparta, British Swordsman in Camulosadae).