as long as you finish the roman campaign it unlocks all the civilizations.
just got finished with the (long version) of the campaign as the julii, took me around 15 total hours or so.
and now im playing as greece... this is great!!!
there infantry are annoying though :/ cant figure them out..
all i understand about them is that when they are standing still they will kick ass, but if they are moving while attacked from the front... they die like they are nothing...
confusing me.