Planescape

Planescape is a campaign setting for the Dungeons & Dragons fantasy role-playing game, originally designed by Zeb Cook. Planescape "solidified the Great Wheel cosmology that began in 1e and would later be reinstated in 5e as the dominant of three theoretical models". Planescape links other Dungeons & Dragons worlds via inter-dimensional magical portals.

The game is set in Sigil, the "City of Doors", a torus shaped city/world that rests atop the Spire. There is no sky, simply an all-pervasive light that waxes and wanes to create day and night. Sigil cannot be entered or exited save via portals. Thus, many call Sigil "The Bird Cage" or "The Cage." There are fifteen philosophy-derived factions. Each of these factions is based on one particular belief system; one faction's beliefs make them enemies while others make them allies.