The Nameless World
The hundreds of worlds of the multiverse are commonly referred to as “spheres” instead of “planets” due to how their physical infrastructure operates versus that of our universe; each of the spheres is composed of concentrically positioned layers of planar sub-spheres, with the upper and lower boundaries demarcated by phenomena known as “gravity hills”, with the Prime Sphere (a.k.a. the Material Sphere) sitting outermost, before the transition into non-local planar layers.
Movement between the spheres is possible using any number of methods, but is considered to be notoriously difficult, and thus is not commonly undertaken by entities without significant levels of deific power, or those moving through Sigil. Working through the planar sub-spheres of a sphere is notably easier, however, assuming one has a well-plotted course or knows the location of thin enough places between the sub-spheres to move through.