Most early civilizations grew up around river valleys where large populations could be supported by surplus food. This surplus allowed a small percentage of the population to pursue facets of civilization other than agriculture. These budding civilzations were under constant pressure from the surrounding people. When the conquerors were assimilated, they became less warlike and, in turn, they were conquered. This cycle was repeated numerous times.
I had this history in mind as I made this map. The four keeps on the plateau around the upper river represent the Barbarians. They have good supplies of iron and stone but limited grassland. The keeps in the valley represent civilization. They have limited access to iron and stone but abundant grassland. I designed this map for two teams of four each but it can work as a free for all.