Hemisphere cosmology
Part 1: The mundane world and the Feywild
The mundane world is a hemisphere, as any child knows. The other half of the world is the Feywild—although, since the Feywild is unstable in all ways, it’s impossible to say whether it too forms a hemisphere, or any other shape.
The mundane world thus has five fixed points, as opposed to the two poles of a sphere: North, South, East, West, and the furthest spot from the Feywild, which is generally called Noon. These are referred to as Points, not poles: Point of East, Point of Noon, etc. It’s a common convention to orient maps with Point of Noon at the top, and the Feywild at the bottom, but this is confusing to people raised on spheres.
Relative to the mundane world, the Feywild is the otherworld—literally the world of Otherness—but it’s also a real, existing place; the PCs could walk there, if there didn’t happen to be a large ocean in the way. In addition to this, the mundane world is in some places pierced by holes like gateways, that lead directly into the Feywild—and then, sometimes, directly back out into the mundane world. These holes in the fabric of the material world are called fey doors. Sometimes travelling through a fey door takes you to the Feywild; more often, it leads to some other part of the mundane world, so that the traveller moves through the Feywild without ever actually being there.
Some scholars—most famously, the esteemed research wizard Iacopo Scutato—suggest that the Feywild contains all other planes and worlds. Heavens and hells, in this worldview, are merely other names for parts of the same chaotic expanse. However it must be noted that the influential goblin philosopher Nchasgh Avhiusischsk (known above ground as Hyacinth Parabola) disagrees with this.
Part 2: The elemental domains
Whether through linguistic convention, or due to the actual nature of the universe, inhabitants of the mundane world tend to speak of the elemental domains as being located deeper within the world. The mundane world is made up of those elements, after all, so it makes sense to travel to their homes by moving further into the true nature of materials. The four elemental domains lie deep within matter, in some spiritual way.
Some say that there’s a further, fifth elemental domain, the quintessence, where the four elements are themselves reduced to the single substance that makes up all physical matter. This substance, vibrating at different speeds, forms all the physical phenomena we know. The quintessence, as both a place and a material, is motionless; it’s literally matter without motion.
There is no agreement on whether the quintessence exists, or if it does, whether mundane people could survive there without themselves being reduced to quintessence. The only sources for its existence are folktales of heroes travelling to the quintessence via a tunnel through the center of the world. In most of these stories, the mouth of the tunnel is at Point of Noon, the point of the mundane world furthest from the Feywild.
Part 3: The elemental domains and the Feywild
Although the Feywild adjoins the mundane world, and the elemental domains are within it (spiritually speaking), the Feywild also touches all the elemental domains. It’s hard to diagram, but it’s true. This is the primary reason for theories that the Feywild is, if not the container for all other worlds, then at least a connective space between them all. Fey people can pop up almost anywhere, it seems; much more rarely, people from other realms turn up in the Feywild.
Part 4: The celestial spheres
In opposition to the elemental domains, people talk about travelling outward to the spheres called “angelic” and “fiendish.” Where the elements are integral to the mundane world, the celestial spheres are wholly removed from it. Nor is there any clear hierarchy between celestial realms: some are inhabited by completely alien beings who care nothing about us; some more closely resemble, and have more contact with, the mundane world. Some of the latter are called heavens, and some are called hells. They have various natures, and their inhabitants have various priorities.
It isn't known for certain whether the Feywild touches the celestial spheres, but all indications are that it does not. Between the mundane world and the celestial spheres, communication and travel are rare, but it seems to be completely impossible from the Feywild. They are completely different types of Other. Hyacinth Parabola conjectures that the Feywild is the realm of physical matter in chaos; the celestial spheres are non-material planes.
Part 5: Other mundane worlds?
It is conjectured, but not known, that there are other mundane worlds—stable worlds made of physical matter—beyond this one. What does “beyond” mean? Are they located above the sky, amongst the stars, so that you could fly there given enough time? Do they occupy the same space as this world, but in another dimension? Does the Feywild touch them as well, or do they have their own version of the Feywild? This is all mere conjecture.