This article describes the planes, their relationships to one another, and the layout of the material plane.
The Material Plane
The Central Beacon, Eua
The center of the material plane is Eua, the sun. Unlike Earth's star, Eua is not a ball of fusing hydrogen. It's a sphere of condensed , light-and-fire-aspected anima fed by the Great Mechanism in the Celestial City. As a result, solar prominences, sun-spots, and solar flares are omens of disorder in the realms celestial. In size and output it is very similar to Sol, however.
The Crystal Shell
The entire setting, including all the planes, is bounded by a crystal shell about twice the distance from the star (Eua) as Earth's orbit is from Sol. Whatever may exist outside is cut off by this shell. In the material plane it is an actual shell made out of some crystalline material that is tougher than anything found elsewhere. Despite its toughness, creatures (or the thoughts of such strange creatures) from the Dark Beyond slip through, with varied results on the material plane.
Planets in the Material Plane
There are 4 planets orbiting Eua in perfectly circular counter-clockwise (as seen from above) orbits. Closest in is Primus, a scorched rock. It is said that creatures from the elemental planes of Fire vacation there. Second and third are Secundus and Tertius--twin planets orbiting each other at such closeness as to share an atmosphere. One is dominated by water and the other by earth. The orbital radius of the double planet is twice that of Primus
Fourth and last is Quartus, orbiting four times as far from Eua as does Primus (1 AU). There once were three moons, but the least moon was pulled down by the first shamans to end the Age of Wizardry 10,000 years ago (roughly). The others settled closer into its place. The moons are Quella, often called the Blood Moon for its red hue, with a period of 32 days, and Teki, the Bone Moon with an 8 day period. Quella is much larger, so they appear approximately the same size (1/2 the angular size of Earth's Moon) in the sky.
Quartus is where the events of the setting take place (with rare exception). Inhabited by the spirits of natural things (including the fey) as well as mortals, it is a place of adventure.
Artist's Depiction of the Material Plane (showing the Abyssal orbit)
The Plane of Shadow
Surrounding Quartus is the transitive, liminal plane of Shadow. The Shadow is the home of the dead--all but the strongest spirits (or those devoured by demons) awaken here and live out a shadow of their life on Quartus until they fade away or are consumed by the predators here. Access to any other plane from Quartus requires passage through Shadow. Although it is transitive, space and time here are malleable, shaped by the intents and wills of strong living beings.
The Inner Planes
There are 12 transitive inner (elemental) planes. These occupy fixed segments of the Crystal Shell, radiating outward from the center. As you progress inward toward Eua, the planes become more energetic and chaotic. At the outside where Quartus orbits the four primalities (Fire, Earth, Air, and Water) merge and mingle to form the 12 elemental planes. Since Quartus's axis is perpendicular to the orbital plane, seasons are caused by the varying influences of the elemental planes it passes through on its orbit.
The year starts with Quartus entering Clay from Mud; summer begins as Quartus transitions from Coal to Lava. Autumn begins as Lightning turns to Smoke; Winter as Cloud becomes Ice.
Artist's Depiction of the ordering of the Elements
The Abyss was designed as a prison for the Nameless and his core followers during the Dawn War between the Primal Potentialities. After the Cataclysm, when the Nameless and the Potentialities were fully absorbed into the planar structure, the Abyss became inhabited by those beings whose desires, goals, and/or methods were abhorrent to the current order of things. Creatures who exist there subsist on the dregs of energy they gather from the Material Plane, often by consuming mortal souls. The Demon Princes, rulers of individual demesnes they have carved out, hold the power here.
Physically the Abyss is a small (about 2 Earth-radii) spherical void that orbits Eua in the space between planes. It's orbit is not simple--it's shaped like a clover-leaf. Twice a year (at the Equinoxes) it draws to its closest metaphysical distance to Quartus and demonic summoning is at its peak.
The Astral Plane
The Astral Plane is the home of the Great Mechanism that regulates all the planes. It pumps anima (the stuff of souls, created when living beings grow and develop) around to the various planes, allowing the inhabitants to survive and maintain the Dream. The center of this Mechanism is the Celestial City, home of the the Celestial Host of angels and the Core. Located at the center of the Dream, "above" (metaphysically) Eua, it pumps anima into that great furnace. The The Heavenly Congregation (the gods) and the Diabolical Families (the devils) live and work in various realms located within the enormous expanse of the Astral Plane. They all are engaged in the oversight and maintenance of the Dream, subordinate to the Great Mechanism and the Four who guide it. Between these realms swim strange creatures--radiant dragons, astral sharks, fish-like creatures, etc.
The Dark Beyond and the Stars
The Dark Beyond is the home of alien creatures, some are benign and some are hostile--all are powerful and terrifying. One particular faction are the Awakeners, formless beings of thought that hate all material existence and try to subvert it and destroy it. The angels do fierce battle against their minions as they leak through the Crystal Shell. There are other entities out there in the endless dark, but little is known about them.
The Dark itself is described in On the Dark Beyond.
The stars are glimmers of other worlds distorted by the Crystal Shell and the intervening space between.