7 culture system
From TWC Wiki
It is possible to add a seventh culture to Barbarian Invasion and Rome - Total War games.
Dol Guldur did the research work to prove this and has written an extension tutorial on the subject see:
THE COMPLETE GUIDE TO CREATING A NEW CULTURE IN R:TW-BARBARIAN INVASION
(and Making All Your Modding Dreams Come True!) posted at the Org.
Or the Wiki version of the tutorial here.
There is a discussion thread for this tutorial here.
