I don't think that's the case Q-Meister, precisely I think you don't get the option to switch to civs that are born near you in the time distance axis not in the land distance axis. So in this case Greece and Rome are both classical civs. Middle Ages civs are grouped in different spawn groups. These also affect starting buildings. Just my theory btw, but I don't think you can switch to the Spanish as the Vikings and they aren't near.