Cultural event
Populations settle down around key resources and defendable positions.
The largest and most successful clans began to build settlements and more permanent structures on their most valuable land. They learn to toil the land and keep animals to sustain themselves year-round. The idea of borders and land ownership starts to be introduced to those powerful enough to take advantage. Major land disputes occur regularly and the worlds crudely defined land borders shape-shift from year to year. All this conflict resulted in towers and other primitive defensive structures being built.