Sigil
Sigil, aka the Cage and the City of Doors, is the cornerstone of the Planescape setting in Dungeons & Dragons and perhaps one of its most iconic planar traits.
Sigil is a massive extraplanar metropolis that traditionally exists in the infinite sky of the Outlands, the True Neutral Plane, floating atop the infinitely tall Spire at the center of the Outlands. As its moniker of "the City of Doors" suggests, Sigil is absolutely lousy with portals throughout the Multiverse, allowing access to theoretically every single planar location and prime world that exists. Fortunately most of the portals require very precise keys to open, are there for a short while, and often both. This makes it the grand melting pot of the cosmos, an interdimensional port-city where everything ends up going to and eventually ends up coming from. Sigil is constructed in a torus shape. Essentially, picture a donut with a city built on the inside.
Sigil is ruled by the enigmatic Lady of Pain, a godlike entity who has but a single law; Don't Fuck My City Up. Actual day-to-day operations are left to the inhabitants to oversee.
Sigil's native denizens, known as Sigilies, are somewhat infamous for their arrogance, sort of the stereotypical Big City Slicker looking down on the rural country bumpkins - which, in their mind, is pretty much everyone who doesn't live in Sigil, but especially Prime Material denizens. This does occasionally bite them, such as when a "Clueless" turns out to be an epic tier wizard who promptly vaporizes the Tiefling street gang trying to mug him.
Places of Interest
The Armory
The place where most of the weapons of the city are stored and made. The Doomguard run the place, because the use of weapons propagates entropy. The building is some 600' by 800', with the barbed construction around it reaching to almost 500'. The main building holds the storage for weapons and warmachines, the forge to make new ones, workshops to replaced damaged goods and the offices to buy and sell. The four corners hold square towers with smaller forges at the center, but there's more to them than that. The dark is that each of the towers has a portal to each of the Doomguard's four citadels in the Negative Energy Planes: one portal per citadel. During the Faction War the place was messed up pretty bad, and Vecna's romp in the city all-out leveled the place.
The City Barracks
Sigil doesn't have a standing army. But to be fair it doesn't really need one. Between the mix of volatile elements within the city, some of the very powerful people living there and the Lady of Pain herself, Sigil is more than capable of defending itself. But for everyday law enforcement a base of operations is needed, so Harmonium has set up kip in the barracks. The barracks are a square, squat building 360' on all sides and 60' tall, despite having only three floors. Most of the space is taken up by the large central courtyard used for drills and exercises. The building itself consists of four towers 60' in all directions, with four long hallways connecting them. The lower floor holds dormitories for the lower ranks (segregated by gender) and various classrooms in which The Law is taught, as well as training rooms, the mess hall, library and two auditoriums. The upper floor holds more classrooms, the records rooms, the quarters for officers as well as the quarters of Factol Sarin, his wife and their nine kids.
The City Court
The City Court is where all legal matters are settled. Many a criminal is hauled in front of the court to stand trial and be sentenced, often without a good end for the suspect. Most crimes are solved with the payment of a fine or in one of the lesser courts, and only the most spectacular or heinous cases (rare) and appeals (even rarer) are held in the opulent grand courtroom. Being such a center of law, it's obvious that the courthouse is also the base of operations of the Fraternity of Order. The main building is an L shape of a rough 100' by 150', with a tower of 40' on all sides besides it. The main building is two stories containing the grand courtroom, record rooms, a few smaller courts as well as lesser buildings and workrooms and dormitories for aides. The tower contains several additional smaller courtrooms and support, as well as offices for the faction.
The Civic Festhall
Despite what your experience with other settings might tell you and what the reputation of the Society of Sensation might be, the Sensorium is not one of those kinds of festhalls you keep reading about in AD&D. A cylindrical tower that's a good 1000 feet tall including all the spikes sticking out of it standing on grounds some 1000' by 750', the front of the Civic Festhall carries a somewhat ghoulish decoration displaying stylished eyes, noses, ears, mouths and hands to celebrate all the senses. The tower itself is made of the most wonder, opulent, intense and interesting materials: touching the walls made of varied types of stone and unique kinds of wood is encouraged. Half of the main tower's ground floor is a series of quarters for the factions as well as classrooms and lecture halls. Part of the grounds housing a number of theatres for all sorts of shows. The other half of the ground floor is taken up by the Sensoriums: the libraries of the senses. There's two of them: one for Faction-only use and one for public access. Each take up a quarter of the full surface of the building, going up well over 20 floors. Here people ran relive the experiences of others, sharing their senses to experience something in a way you could otherwise only experience yourself. Experiencing something here costs 10 gold for the basic package, and the place is always full despite being open 24/7. If you've got something new to offer that the Sensates don't have already you can expect to receive 500 gold to pass on this experience. Recording it merely makes a copy of your memory and can take anything from 1 hour to 1 day, depending on the complexity of the memory. The remainder half of the tower is filled with rooms where one can experience just about anything, from the smells to sights and tastes and more wonderful things still. The places is filled with taverns, museums, craftsmen and other services and diversions that one can barely imagine. At the center of the ground floor, accessible to only a few Sensates, is the faction's greatest secret: a series of portals leading to just about all of the Outer Planes and Inner Planes, granting the Sensates unmatched maneuverability across the multiverse.
The Gatehouse
The Gatehouse is an almshouse, sanitarium, orphanage, asylum for the criminally insane and soup kitchen all in one, as well as the headquarters of the Bleak Cabal. The building and grounds cover a semicircle some 800' wide and 3 stories tall. The left wing holds the almshouse on all three stories, with the right wing holding the insane asylum on the first two floors and the orphanage on the third. There are two other wings to the back: one sticking out at 90 degrees from the main building and one at 45 degrees towards the right. The first is the wing for the mad Bleakers who are under the effect of the Grim Retreat, while the other one holds a the criminally insane. After the Faction War the Bleakers disbanded but continued their work here, because who else is going to run the soup kitchens?
The Great Foundry
Headquarters of the Believers of the Source, the Godsmen run the place part to generate revenue, part to build character. The place makes all sorts of metal objects and tools for the people of Sigil on a working-class budget. The compound is some 700' at its widest point, with the main foundry reading about 200' into the sky, belching out its dark fumes. Several towers stand together to melt metal and make it into bards, sheets, wires and use molds. About half the compound is taken up by a storage and processing yard, bringing in rough materials and transporting finished products. As with any old-timey steel foundries the place is a horror for your health, and frequent healings are advised if you spend a lot of time in the area. Problem is that if you work in the place you probably can't afford the healing to not die of several kinds of lung disease.
The Great Gymnasium
The people of a dirty city like Sigil will want to get clean now and then, and the Great Gymnasium is a great place to do so. A 160' by 100' building, the Gymnasum can be used for all sorts of physical activity to master one's self, as befitting for the Transcendent Order. The main attraction of the large exercise field lay a trio of pools: one cold, one warm and one hot. Around the pools is plenty of space of all sorts of physical exercise, from fighting to athletics. Around three of the walls lay a large number of roomes used for steam baths, massages, more private baths but also rooms for the arts: painting, sculpting, dancing and music are all practiced here for their ability to perfect the self. The second floor houses a wide and narrow meditation chamber for those looking for peace and quiet to contemplate. The third floor is faction members only and houses the Cadence of the Planes chamber: a sensory deprivation room filled with a levitation field and magic to keep the place dark and quiet. Here a Cipher can come into contact with the Cadence of the Planes, the rhythm of the planes which connects them all. The Gymnasium does not hold any offices for the Ciphers: many of them have their own places to stay and Factol Rhys lives wherever she ends up.
The Hall of Records
The closest thing Sigil has to a city hall, the Hall of Records provides all sorts of civil services... for a price. Citizens are going to pay out the nose for the services provided here, both in official prices and below the table bribes. The Hall stands in a campus some 1000' wide containing six buildings: halls for property and census records, the faction hall and faction dormitory, a special academy for training promising Takers. The central building is a monstrous 30 story tower, 240' at the base and tapering up to a rough 200' at the top. In the aftermath of the Faction War the place was raided by the people of Sigil and almost all records was lost, freeing a lot of people of their debts and loans.
The Hall of Speakers
The great debate hall of Sigil, the Hall of Speakers is where policy for the city is debated and voted for. An oval building some 800' long and 400' wide decorated with a spire almost as tall as the building is wide, the Hall of Speakers is both a public building where the law gets made and also the headquarters of the Sign of One. The most notable room here is the Speaker's Podium, where people of all kinds can plead their cases in order to get something done. Getting onto the list of speakers is difficult, and getting onto and keeping your enemies off of it is a battle in and of itself. On the far end of the Speaker's Podium is the tomb of Rilith, the founder of the Sign of One. Quite a few Signers are here at all times, concentrating on the urn of ashes because they believe that without them doing this it'll vanish, and with it the faction itself. Despite being the headquarters of the Sign of One there are few guards posted around and one even has a decent shot at getting to the Factol's personal quarters, but given the nature of the Signers there's not a lot to steal here.
The Morturay
Where the dead go after they die. The Dustmen have set up shop here, and set out to collect the bodies in Sigil and bring them here. The main building is a semisphere of some 200' wide, with a motley assortment of towers connected to the main building via tunnels and corridors. The place has vaults to inter the dead, and also has portals various Prime Material planes and to Arcadia, Mechanus, Acheron, Ysgard, Limbo and Pandemonium (aka the six planes on the Whatever Neutral axis that aren't the Outlands); all destinations being near notable burial sites. The second floor is where a lot of the work gets done, and both Factol Skall and his right-hand man have their offices here. Near the top are portals to the Elemental Planes, and in some of the larger towers there are laboratories and storage rooms. At all times the Morturary is filled with the Dead and undead alike, with some of the more sensitive areasy being guarded by powerful creatures such as juju zombies. The Morturary is also known for being the first level in Planescape: Torment.
The Prison
The prison is not a nice place, and especially so since Factol Nilesia of the Mercykillers took charge. The prison looks a lot like the Barracks, except larger: being 1100' wide on all sides and nine stories tall, with most of the room being reserved for the cells of inmates. The entrance holds the rooms of the high-ups: Factol Nilesia and her retinue have their rooms here. The central courtyard is part drill field and exercise grounds, and holds a massive set of gallows used for the daily hangings that prisoners are forced to watch as part of being let out to air. The cells are dark, damp and cold with little room for prisoners to move around: they're occasionally called to the dreaded cellars to perform work... or get tortured, maimed or executed in accordance to their sentencing. Its maximum capacity is 24000, but with the many executions and disappearances it's never quite full.
The Shattered Temple
The old temple of Aoskar, the god of portals who got his shit kicked in by the Lady of Pain. The ruins are believed to be cursed or still hold the ire of Her Serenity, and most folks will avoid the place. Whatever the case might be, the ruins are the headquarters of the Athar. The place's not much to look at being a 70' wide square with some of the old pillars going up to 100', but the Athar make do. In the center of the temple stands the Bois Verdurous. The Athar Factol can by touching the tree cast any divine spell he wants, and the fruit from the tree can provide the single casting of a spell on the move. But only the Athar Factol can do it: anyone else trying it will have their magic items drained of their power and take d12 damage per turn.