Here's what I use for
Urbis:
"The most commonly accepted theories of the planes of existence state that the Astral Plane holds all the planes together. At certain locations in the Astral Plane the so-called cosmic forces or cardinal points can be found - these (or so it is thought) embody some of the fundamental concepts of reality. As far it is known, it is impossible for living beings to enter these points and return, for the nature of these forces would instantly overwhelm even the strongest magical protections and consume the unfortunate creature utterly. Ten cosmic forces are known to exist: Fire, Water, Earth, Air, Good, Evil, Law, Chaos, Positive Energy (also known as Life), and Negative Energy (also known as Death or Undeath). Each force always has a force in opposition to it: Fire versus Water, Law versus Chaos, and so on.
These forces exert a "pull" on large regions of astral space, similar to the pull a sun exerts on its planets, or a whirlpool on ships. Planes of varying sizes can exists inside of this region, but the "closer" the plane in question is relative to the cosmic force, the smaller and more unstable it will be. However, none of these planes is truly stable (with the exceptions described below), and all will break up into smaller planes eventually. Similarly, small planes will form and grow in size as they absorb other planes within the same region. The planes orbiting the cardinal points of Fire, Water, Earth and Air are often collectively referred to as the Elemental Planes. Each of these planes is "surrounded" by its own Ethereal Plane, which some sages speculate serves as a protective layer for the plane itself.
It should noted that terms like "close" and "far" aren't to be taken too literally when applied to the Astral Plane. While a visitor to the Astral sees it as he would see three-dimensional space, this is an illusion created by his mind - the Astral Plane does not have the same dimensions as the Material Plane does. Rather, "close" and "far" should be taken as metaphors - as a gauge of the affinity any given plane has for the cosmic forces. While planes orbiting a specific cosmic force usually do have a three-dimensional geography, they do not have any regions that are closer to the cosmic force than others - rather, the affinity of the plane for the cosmic force is equal everywhere. While certain magics or the force of will of certain creatures can make a part of such a plain hotter or more evil than others, this is only the result of individual meddling, and does not alter the plane itself in a fundamental way.
There are certain locations in astral space where the pull of two cosmic forces is equal, but in opposite directions. Most of these locations are unstable, which means that any plane at that location would soon be pulled in one direction or the other. However, there is always one (and only one) location between two cosmic forces that aren't opposed to each other which is stable. At these locations, large and stable planes form that contain elements of both of the cosmic forces, but are ultimately dominated by neither. These are practically worlds in their own right, and are host to a variety of creatures. For example, the devils hail from a plane balanced between the cosmic forces of Law and Evil, while the demons breed on a plane that hangs between Evil and Chaos. There are 40 such planes, one for each pairing of non-opposed cardinal points.
Other stable planes exist as well. These are mostly the realms of various deities, who can prevent their dissolution through their godly willpower alone. The realms of weaker deities can often be found in the vicinity of one or several of the cosmic forces, which allows them to siphon off some of their energy, but having a realm in such a location would be too limiting for the more powerful deities, and consequently their private planes are far away from any of the cardinal points. This might help to explain the general tendency towards neutrality among the most potent gods. Some sages speculate that these gods might actually attempt to transform their portfolio and themselves into an entirely new cosmic force. While this hasn't happened in recorded history, it doesn't mean that this cannot happen...
It was thought that the Material Plane exists at the "center" of existence and is "equidistant" to all the cardinal points, but new research suggests that this is true only for Rothea. It seems that other planets have their own Astral Planes (reachable from Rothea only through Epic Spells), and their own unique sets of cardinal points. The world of Yethrod, for example, seems to lack a cardinal point of Fire, but has an additional cosmic force of Sound. This revelation threw earlier theories in question that suggested that the cosmic forces were the wellspring of all creation, and made many scholars wonder how necessary the "cosmic forces" are for the actual cosmos.
Research is continuing to this day."
But wait - there's more. Not only are there other planes to visit in
Urbis, but other
planets as well - something I am missing from many traditional fantasy world (with the exception of spelljammer). After all, why not use those funky teleportation spells to get somewhere
really far away?
"Other planets can be seen circling in the night sky above. Sometimes, mighty archmages have visited them, bringing back tales of strange and fantastic places if they bother to tell about their exploits at all.
And sometimes, the inhabitants of these far-off worlds come to visit this one.
Magrith
A hot and wet world covered by dense jungles and deep oceans.
Known inhabitants of this planet include carrion crawlers, chuuls, digesters, ethereal filchers, ettercaps, fungi, othyughs, phantom fungi, purple worms, shambling mounds, spider eaters, stirges, and tendriculous.
Rothea
The home world of humanity and many other intelligent races besides. Its geography and people are detailed elsewhere. Its solitary moon, Uluth, appears lifeless to the naked eye, but explorers who braved its airless surface tell of creatures of elemental earth who have made their home there. Some tales tell of vast underground cavern complexes with thriving ecosystems, but these reports must be considered apocryphal at bet.
Also noteworthy are two large rocks with a length of approximately one hundred miles each that are located at the points where the gravitational pull of Rothea and Uluth cancel each other out. These are commonly called "Uluth's Herald" (or "the Herald") and "Uluth's Servant" (or "the Servant), and appear approximately four hours before (in the case of the Herald) or after (in the case of the Servant) Uluth in the sky. Some diviners have reported seeing structures on the surface of these moonlets, pointing to inhabitants, but the truth behind this remains a mystery.
Yethrod
A wet world with a dense atmosphere, Yethrod is perpetually shrouded in mist. Yethrod has almost no natural elevations like mountains that break through its dense cloud cover. Instead, it has massive plants that are many miles high and that have been dubbed "world trees" by explorers, whose upper surface reaches beyond the clouds and hungrily drinks all the surface it can get. Its massive seeds float as well, thanks to lighter-than-air gases that accumulate in its interior. The seed trails a long root behind it that reaches all the way to the ground. When that root finds a locale that is suitable as a base for the world tree, it fixes itself to the ground and grows until it becomes the massive trunk of an adult world tree.
Below its cloud cover, the atmosphere of Yethrod has a very low visibility. This has caused the animals of this planet to have underdeveloped visual organs - indeed, eyes are often entirely absent. On the other hand, they have a keenly developed sense of hearing, and most beings orientate themselves by sonar. Some beings have developed their sonar to such a strength that they can even rend flesh with it, and use it for hunting.
Know inhabitants of this planet include destrachans and yrthaks.
Surtus
A large, cool and dry world without any major bodies of water. Its wind-swept surface has a high content of iron, which gives the entire planet a rust red color.
Known inhabitants of this planet include ankhegs, basilisks, behirs, bulettes, delvers, displacer beasts, frost worms, manticores, remorhazes, rust monsters, and umber hulks.
Calturus
The outermost planet in the solar system, Calturus presents a cold, airless surface to the observer. Its interior is still heated by volcanic activity, however, and all kinds of loathsome and slimy creatures thrive in the wet, subterranean cave systems.
Known inhabitants of this planet include aboleths, cloakers, darkmantles, gray renders, gricks, mind flayers, and ropers."