As I understand it, what moonburn referred to was the occasional EB multiplayer tournaments, which are played with strict historical rules regarding the composition of armies by faction and what moves are not allowed. For example, non-steppe factions are limited in the number of archers they can field, for the sake of balance, and only Carthage is allowed to have more than five units of mercenaries in their armies (Or something along those lines). Also, actions such as phalanx stacking and charging your horses/elephants through the back of your own lines are considered illegal moves. Of course, these only apply to the specific tournaments, and not to general MP play, in which people are free to play as they wish.