Safety Above, Danger Below
The surface of the world of Raeluna is a wildly dangerous place, a wilderness of blasted lands, creeping jungles, and crumbling ruins. Sharp-toothed beasts and strange monstrosities abound, and to travel the broad wilds of the world is the height of both bravery and foolishness. Pockets of safety from these evils exist on high mountaintop retreats or in mightily constructed citadels - but the bulk of humanity lives in the air.
By ancient arcane magics Islands of rock and earth hang untouched by gravity in the skies of the world. Everything from small boulders to square miles of forested hills to entire mountain ranges sail predictable routes far above the mindless savagery of the various 'Lowlands'. On these Islands humanity has built its existence in the enormous ruins of an extinct Ancient Race, dismantling uncomfortably large halls and monuments and creating their own civilization among the mysterious remains.
The Islands are intermittently connected by large teleportation portals known as the Gates of the World, each part of a ruin left by the Ancient Race, and each protected by at least one statuesque and inscrutable Gate Guardian. The Gates open and close with patterns presumably determined by their long-dead creators and with the contemporary help of the Arcanists, a society of human magic users with some understanding of the Ancient's arcane mysteries.