I was thinking about the fact not all the civilizations of history are based on city systems. It's so strange to me to see Iroquis or Mongols cities!
If Territory, cities and culture borders are splitted, it would be possible to think about nomadic civilizations, with the limits and the advantages of nomadic cultures, and with the possibility to change (but not easily) from a system to the other. In this manner, an invasion from china to mongolia would cause the MOVEMENT of the mongol civilization, with the limit of the new occupied territory. At the same time the new occupied areas of china would be in part of mongol's culture borders (decreasing during time) and the borders of Mongol's territory would just be shifted. In that way you would be able also to simulate the great migrations wich were cause of many great changes in History.
If Territory, cities and culture borders are splitted, it would be possible to think about nomadic civilizations, with the limits and the advantages of nomadic cultures, and with the possibility to change (but not easily) from a system to the other. In this manner, an invasion from china to mongolia would cause the MOVEMENT of the mongol civilization, with the limit of the new occupied territory. At the same time the new occupied areas of china would be in part of mongol's culture borders (decreasing during time) and the borders of Mongol's territory would just be shifted. In that way you would be able also to simulate the great migrations wich were cause of many great changes in History.