One great thing about basing a campaign on Earth, I can use old maps! Heck even Google Maps is useful. While working on my Mystic Earth campaign i was wrting up the area now know as France. Based on the time frame I am using the Merovingians are still the rulers of this area. I Google the Merovingian kingdoms and wala!
For Paris in the mid 6th century i have to make do with a map from the 11th century.
Still looks fantastic for a game map even if slightly anarchanistic.