Planescape Canonical Campaign
The Outlands is the plane between the Outer Planes. It is the plane of neutrality, incorporating a little of everything and keeping all aspects in a paradoxical balance—simultaneously concordant and in opposition. The plane has varied terrain, with prairies, mountains, and shallow rivers.
The Outlands is circular, like a great disk. In fact, those who envision the Outer Planes as a wheel point to the Outlands as proof, calling it a microcosm of the planes. That argument might be circular, since the arrangement of the Outlands inspired the idea of the Great Wheel originally.
Around the outside edge of the circle, evenly spaced, are the Gate Towns – sixteen settlements, each built around a portal leading to one of the Outer Planes. Each town shares many of the characteristics of the plane where its gate leads. Planar emissaries often meet in these towns, so it isn’t unusual to see strange pairings, such as a celestial and a fiend arguing in a tavern while sharing a fine bottle of wine.
Given the fact that you can ride a horse in the Outlands from a heaven to a hell, a planar-themed campaign can be set there without the need for planar travel. The Outlands is the closest the Outer Planes come to being like a world on the Prime Material Plane.