Constant struggle between the primordial forces of the wilderness, and the taming effects of civilization. Outside of the few major cities of the world, of which Overlook is one of the greatest, the wilderness stubbornly defies all attempts to map it. A forest 10 miles on a side may contain 500 square miles of land. Walking clockwise around a hill will lead you to a completely different place on the other side than if you go the other way around.
The tribes of nomadic peoples are used to this strangeness, and navigate by very carefully following the landmarks they know. Civilized folk tend to stay within sight of their villages as much as possible. A city the size of Overlook extends its taming influence several miles out into the countryside, to the farmlands and hamlets that feed the city.
The web of portals linking the cities are the lifeline that keeps civilization in contact with itself. Two cities so far apart that travel by land or sea is impossible can be seconds apart by portal. In fact, some destinations are so far from any known lands that it’s believed they may be on another world entirely!