Ideas Given Form
Residing in the conceptual realm of the Astral Plane, just beyond the outskirts of the Elemental Planes, are the sixteen Outer Planes. Each one embodies an idea (conveniently simplified and represented by the Alignments), and each plane is likely governed by the deities that represent these ideas. Creatures who serve these deities, or who themselves sufficiently represented such ideas in life, might find themselves in one of these planes after death.
Also in the realm of the Astral Plane, are the Outlands, a vast field with sixteen towns based at its edges. Each town is a settlement that exemplifies one of the Outer Planes, and is a gateway to that plane. Lastly, hovering above the immensely tall landmark known as the Spire that stands at the Outland's center, is the ringed city of Sigil, City of Doors. A major hub for interplanar travel, the city contains countless portals to every single other plane. According to Sigil's denizens, they are the self-proclaimed center of the multiverse.
The ruler of Sigil is the mysterious Lady of Pain, who reigns from her seat of power known as the Throne of Blades. Her power alone controls all the portals in the city and prevents all deities and archfiends from ever entering it.