Before the Romans conquered Gaul, the Gauls lived in villages organised in wider tribes. The Romans referred to the smallest of these groups as "pagi" and the widest ones as "civitates". These "pagi" and "civitates" were often taken as a basis for the imperial administration and would survive up to the middle-ages when their capitals became centres of bishoprics. These religious provinces would survive until the French revolution. During the Roman Empire, southern Gaul was more heavily populated and because of this more episcopal sees were present there at first while in northern France they shrank greatly in size because of the barbarian invasions and became heavily fortified to resist the invaders.