Sigil, the City of Doors

Sigil is located in the Outlands, hovering above the immensely tall landmark known as the Spire that stood at the plane's center. From the Outlands, it is possible to see Sigil hovering atop the Spire as the mountaintop vanished into nothingness.
Sigil is shaped like the inside of a torus. According to official measurements by the Harmonium, the city's primary diameter―that is, the ring's diameter―is 5 miles , with a circumference of 20 miles . Later measurements put the primary diameter at approximately 6.4 miles (10 kilometers), with the same circumference as the Harmonium's measurements, and a secondary diameter―the ring's thickness―of approximately 1.5 miles . Despite these attempts at measurement, the city did not have a fixed size and its dimensions could be changed at will by the Lady of Pain.
The city's location and geometry is considered by most sages, in particular the inhabitants of the Material Plane, to be impossible. Although all magic is completely impeded at the center of the Outlands, it worked normally within Sigil, with the exception of spells that involved planar travel. There were many hypotheses that tried to explain Sigil's location and existence. Although they varied wildly, one of the more popular explanations posited that the Lady of Pain is behind the city's existence and properties.
The city did not fill the entire inner surface of the torus, but just the outer portion of the ring. For that reason, the city is recursive only in one direction, along its major circumference. Even though it is not a completely closed surface, it is impossible to see outside of the ring from any point within the city. If one looked up, the far side is visible, as the gentle curvature caused any point in the city to resemble the bottom of a valley. The edges of the ring were lined with solid buildings that has no windows outside, so the only way to try to see what lay beyond the edge is to climb a rooftop. Those who did reported that there is nothing to see beyond the edge―not empty space or a vacuum, but nothing at all. Those who jumped over the edge disappeared into a random plane.
As a result of this unique geometry, Sigil has no sky. The city is kept lit by an intrinsic luminescence of its air, which waxed and waned creating the appearance of a 24-hour day-night cycle. The brightest moment in the cycle is referred to as "peak" and the darkest point is referred to as "antipeak". Time is measured by counting down the hours before peak (b.p.) and incrementing them after peak . As a result, most of the time the city is illuminated only by a hazy twilight. The equivalent of full daylight only lasted for about 6 hours, from 3 b.p. to 3 a.p., so even creatures that were sensitive to bright light were comfortable in Sigil most of the time. During the six darkest hours of the cycle, light coming from lanterns on the opposite side of the ring gave an illusion of a sky filled with stars. Clocks in Sigil did not have numbers and were marked with 24 1-hour segments, with the daytime hours and peak on top and night hours and antipeak at the bottom.
The city is crowded with buildings that squeezed over each other with very small living spaces. New buildings, courtyards, and streets were constantly being built, changing the city's landscape and turning old structures into underground crypts. Sigil's architecture is marked by its iron spikes and bladed fences, serving both as protection against intruders and as a stylistic choice. Stone gargoyles were also typical decorations.
Weather on Sigil alternated between foul-smelling smog, frequent chilly rains, and somewhat clear weather. The haze from the city's myriad chimneys is a near constant, often reducing visibility to about 10 yards (9.1 meters), or even down to 5 feet (1.5 meters) in the worst cases. When it rained, it is common for the water to mix with the walls' and the air's impurities and turn to a brown drizzle. After it rained, the city air cleared out for a while, with light breezes and a pleasant cool temperature.

Cosmography

According to its own inhabitants, Sigil's central position in the Outlands, at equal distance from the each of the gate-towns that led to the Outer Planes, put the city itself at the center of the planes, at least according to some interpretations of the Great Wheel cosmology. Most sages agreed that, since the multiverse is infinite, there is no true center, but nevertheless Sigil is generally acknowledged as one of the most important places in the multiverse.
It is impossible to walk into and out of Sigil from the Outlands or any other location. It is also impossible to enter or exit the city, or even observe it, via spell, magic item, or any creature's innate abilities. Although spells like plane shift, gate, and astral projection were incapable of entering or exiting Sigil, the city is not completely cut off from the Astral Plane, so spells such as raise dead, which required access to the Astral, still worked. Teleportation within the boundaries of the city also functioned normally. Like all locations in the Outer Planes, Sigil is cut off from the Ethereal Plane.
The only way in or out of Sigil is via its innumerable portals. Any bounded opening (a doorway, an arch, a barrel hoop, a picture frame) could possibly be a portal to another plane, or to another point in Sigil itself. In addition, portals could be permanent or temporary, linking to fixed or shifting locations. Thus, the city touched all planes at once, yet ultimately belonged to none. Those characteristics warranted Sigil the other names it is known for: it is called "the City of Doors" for the sheer number of portals, but is also called "the Cage" for the difficulty to enter or exit the city.

Government

The Lady is an entity of inscrutable motives, but she is known for being swift and merciless when dealing with threats to her and to the City of Doors. Those who crossed her, even accidentally, risked being flayed to death or teleported to extradimensional labyrinths known as the Lady's Mazes, never to be seen again. Tales also existed of would-be usurpers to the Throne of Blades who ended up imprisoned in Agathion, the third layer of Pandemonium.
Although the Lady of Pain did not act directly in the everyday management of Sigil, her will is enforced through a number of servants known as dabus, who simultaneously served as the Lady's eyes and ears as well as keepers of the structure of Sigil. Like the Lady, the dabus did not interact much with Sigil's inhabitants or travelers. It si considered wise to leave the dabus be, since antagonizing them incurred the risk of bringing down the infrequent but harsh wrath of their mistress.
The dabus also constantly repair and reshap Sigil, recycling materials from one structure to build another. They were believed to be the only ones who truly understood the inner workings of the city, and some even speculated that the dabus were the true rulers of Sigil. It is believed that the dabus dwelled somewhere in the crypts in the city's underground. Thanks to the Lady's strict forbiddance of open large-scale conflict, Sigil is a true neutral haven to all visitors. It is a location where no wars were waged and even the fiercest opponents, such as an angel and a fiend or a devil and a demon, could be seen sharing a drink and momentarily setting their differences aside.
Nevertheless, Sigil is hardly peaceful; the Lady of Pain did not concern herself with day-to-day crime such as murder or petty theft. Only threats to Sigil, and, by extension, to the Lady herself, were met with direct action by her and the dabus. This lack of direct oversight and the need for personal maintenance of law and order gave visitors and Cagers the feeling that Sigil is constantly on the brink of anarchy.

Services

Services offered in Sigil includ:
Touts, or unofficial city guides. It is highly recommended that planewalkers new to Sigil employed a guide, lest they be taken advantage of or mugged. Since there as no central agency or any regulation of the profession, such guides could be little better themselves, either serving to persuade a traveler to the side of their faction or simply robbing their "customer" once their backs were turned.
Factotums, the city's official guides. Regulated by their respective factions, they guided visitors through not only the city's mazes, but its convoluted politics as well.
Public transportation. Sigil has no horses, so unlike Waterdeep's drays and hire-coaches, its transportation system consisted of sedan chairs that could hold up to two people each. The service did not extend to the entire city, as some dangerous areas were avoided by sedan chair pullers.
Couriers, hired to deliver messages around the city. It is a dangerous job, so the service is not entirely reliable. Light boys, street urchins who offered escorting services via wands of continual light, since Sigil did not possess streetlights. They were familiar with many of the city's intricacies, so they could act as unofficial guides as well

Locations

The Hall of Concordance was an embassy of law, dedicated to the signing and enforcement of contracts. The hall offered the possibility of having the kolyarut engrave a contract onto a sheet of gold that was installed on a marut's chest. The marut then became bound to ensure that the contract was fulfilled to the letter.
Imel's Happy Tongue, an inn and tavern known for its periodic menu specials that featured food from all the planes. The favorite item in its annual "Cheeses of the Planes" was the Torilian Death Cheese.[43]
The Prime Exotics shop specialized in selling small animals from the Prime Material plane, including rabbits, squirrels, hedgehogs, and miniature giant space hamsters.

Wards

Sigil was divided into six wards. Although the boundary between wards was not clearly marked on the city's streets or maps, it was usually easy to identify a ward based on the general upkeep and packing of the buildings, as well as the type of business conducted there. Counter-clockwise around the ring, the wards of Sigil were the following:
The Lady's Ward
The richest and most exclusive section of the city, home to the elites of society and of its government.\
Market Ward
The main location for purchasing goods and services. It was famous for offering items from numerous planes and worlds.
Guildhall Ward Usually counted as part of the Market Ward. Home to the traders, craftsmen, artisans, guild members and other members of the middle class.
Clerk's Ward
An affluent district, home to most of the city's lower-rung bureaucrats and middlemen.
Hive Ward
The slum and the ghetto, home to the poor, the rogues, and the unwanted dregs of the city.
Lower Ward
An industrial district, clogged up with the smoke from the foundries and from the portals to the Lower Planes.
Factions of Sigil

Type

Megalopolis