They're semi-nomadic, moving between the North and spending winters in the more southern regions around the coast. And the Cities are relics of the Warring States age, when the populations were much higher, because they had a very high and plentiful food source, each other. They would eat the dead soldiers or the sacrificial victims, meaning food was much more abundant back then, but now that they have no reason to kill and eat each other, they've turned largely to seminomadism, living in the old cities in the summer and around the coast in winter.