Citadel

The Citadel is a massive space station, constructed over 50 millennia ago in the heart of the Serpent Nebula, close to the star known as the Widow. The nexus of the galactic community, it acts as its political, cultural, and financial capital, housing the Citadel Council, a powerful, multi-species governing body. The Citadel is hailed as being the greatest creation of the Protheans.

Statistics
See more: Codex:Citadel Statistics
 * Total length (open): 44.7km
 * Diameter (open): 12.8km
 * Thickness of Ward arms: 330m
 * Presidium diameter: 7.2km
 * Exterior armor thickness: 13m
 * Gross weight: 7.11 billion metric tons
 * Population: 13.2 million (excluding keepers)

Galactic Standard Time
According to the book Mass Effect: Revelation, the time used in Citadel Space is different to human Coordinated Universal Time in that:
 * A Galactic Standard Day comprises 20 hours.
 * Each Hour comprises 100 minutes.
 * Each Minute comprises 100 seconds.
 * Each second is half as long as a human second.

As a result, a twenty-hour Galactic Standard Day is 15.7% longer than a standard twenty-four hour Terran Coordinated Universal day.

History
The asari were the first race to discover the Citadel, soon after learning to use the mass relays. They were joined shortly by the salarians, and soon after, the volus. As other races were contacted by the asari, or discovered the station independently, they gained embassies on the Citadel and came under the jurisdiction of the growing Council, eventually comprised of the asari, the salarians, and the turians.

Strategically located at the junction of a number of mass relays leading to various parts of the galaxy, the Citadel quickly became a hub of activity. The station was chosen to house the Citadel Council, thereby cementing its importance in the galactic community. It is accepted to be the political and cultural heart of Citadel space, the unofficial name given to all systems that fall under the Citadel species' control.

Even after thousands of years of occupation, the Citadel retains many secrets. The precise age of the station is not known, nor what resilient material it is constructed from. The location of the Citadel's core and its master control unit, regulating systems such as life support and navigation, remain hidden. The most visible, and perhaps intriguing, mystery of the Citadel concerns its caretakers, the keepers. These mute alien creatures maintain the Citadel's vital systems, and can be seen working throughout the station, yet, despite this, little is known about them.

Layout
The Citadel is a massive construct, similar in shape to a pentagram. It consists of a central ring 7.2 kilometers in diameter, from which five arms protrude, each 43.6 km long and 330m thick. The station is capable of closing in on itself, via the convergence of its five arms, transforming it into a long, impenetrable cylinder. However, the Citadel is only closed in times of emergency, and normally the station retains its 'open' appearance. The central ring and five arms are home to the Presidium and Wards respectively; the spaceports are also located on the central ring, as it is the center of rotation, facilitating easier access for ships attempting to dock with the station.

Presidium
The Presidium is a massive, park-like complex containing the offices of the various branches of the galactic government, as well as the embassies of all the races represented in the Citadel. There are also shops, restaurants and recreational facilities, serving the wealthy residents who live and work throughout the sector. The Presidium is monitored by a Virtual Intelligence named Avina, and, unlike the Wards, incorporates an artificial 20-hour day schedule: lights are dimmed, and the holographic "sky" darkened, for 6 of the 20 hours, imitating the familiar day-night cycle.

Citadel Tower
The tower at the center of the Presidium houses the Council Chambers and the main traffic control for the station—any ship within a few thousand kilometers comes under the jurisdiction of Citadel Control. The Chambers themselves are not open to the public and access is only granted to officials and those with special clearance. They are beautifully furnished, with cherry trees and fountains, as well as numerous staircases leading up to the central platform where the Council convenes.

Wards
The 'arms' of the Citadel, known as the Wards, constitute the residential and commercial sectors of the station. Densely populated, housing millions of residents from many galactic species, the Wards are akin to Earth cities such as Hong Kong and Singapore. Numerous skyscrapers rise from the superstructure, sealed against vacuum, as breathable atmosphere is only maintained to a height of approximately 7 meters. Unlike the Presidium, there is no artificial day-night cycle; as a consequence of this, commercial activity rarely ceases, and residents work and rest according to personal need. Barla Von appropriately dubs the area the cultural heart of the Citadel, and therefore of Citadel space. The Wards are policed by the Citadel Security Services (C-Sec), whose offices and custody suites are located in the Lower Wards. Flux is a popular night club, also located in the Wards.

Inhabitants
Duct Rats are poor, homeless children of the Citadel who travel in the ventilation system of the station. They often die by being crushed by the fans; starving to death; or suffocating because of exposure to the open vacuum of space. Often times, they are just never found. Thane Krios sometimes used duct rats, Mouse among them, to gather information on his targets in the Citadel. The keepers are a mysterious, bio-engineered insectoid race that are native only to the Citadel. They are found throughout the Citadel in random locations, and are harmless. They are believed to be created by the Protheans as the caretakers of the Citadel.
 * Duct Rats
 * Keepers

Defenses
As befits such an important galactic location, the Citadel boasts multiple layers of defense. The first is the Serpent Nebula itself: the nebula's composition is such that it is difficult for attacking fleets to navigate, thus preventing them from launching a concerted attack.

Another link in the Citadel's defensive chain is the Citadel Fleet. The fleet is a massive task force consisting mostly of turian vessels but also including ships from the other Council races. The Citadel Fleet, led by the imposing asari dreadnought Destiny Ascension, constantly patrols the space around the station. However, should any aggressor manage to break through the Citadel Fleet, reinforcements from all over the galaxy are only minutes away, due to the network of mass relays located nearby.

If any enemy gets close enough, the station is capable of closing in on itself to form a long, armored cylinder, 25 kilometers in length. The station's hull is sufficiently strong that, even when subjected to the most advanced weaponry available, it would take several days of sustained bombardment to inflict any serious damage to the superstructure.

The station also boasts its own internal security force, known as C-Sec. The 200,000 officers patrolling the station handle everything from customs to hostage negotiations.

The Citadel also has turrets to ward off small craft, such as dropships.

The Last Days of the Protheans
The Citadel was not, in fact, constructed by the Protheans; they were simply the last race to make use of the station before the asari arrived. It, and the mass relays, are the creations of the Reapers, and were built to help facilitate their cycle of galactic genocide. The Reapers were also responsible for the placement of the keepers on the station, in order that the occupying races would not discover the Citadel's key functions. The station is actually an inactive mass relay leading to dark space, designed as an elaborate trap so the Reapers can wipe out the heart of galactic civilization and leadership in a single, devastating strike.

According to Vigil, the last Protheans used the Conduit and traveled to the Citadel to try and break this cycle. However, the Conduit portal only linked in one direction, and Vigil feared that the Prothean scientists, unable to find any food or water on the Citadel, slowly starved to death. Due to the ministrations of the keepers and the long passage of time, it is unlikely the remains of the scientists will ever be found.

During Commander Shepard's visions on Eden Prime and Virmire, very brief glimpses of the Citadel can be seen (namely the Presidium ring from above, and the Ward Arms) presumably as part of the Prothean warning that the station is a dormant trap. However, this part of the vision appears to be too fragmented to be fully recovered, as neither Liara T'Soni nor Shiala mention it.

Trivia

 * The Presidium is a torus, a shape commonly used in science fiction for space stations and starships (including the space station featured in 2001: A Space Odyssey.)
 * Specifically the Presidium resembles the 'Stanford Torus', a vision of a future space-colony, images of which were published by NASA in the 1970s. The design of transport, clothing, seating, and the open urban spaces are taken from a similar portfolio of artists' visions that were published by United States Steel International in the 1960s. (Compare also Arcturus Station.)
 * Interestingly, the Citadel Tower appears to actually resemble a Reaper when viewed from the map.
 * The arms of the station bear a resemblance to an O'Neill cylinder, a type of space habitat proposed by physicist Gerard K. O'Neill, in the 1970s.
 * The role of the Citadel, a space station which serves as the central hub for commerce and diplomacy for multiple species, was possibly inspired by Babylon 5.
 * The Citadel may draw inspiration from the city of Sigil from the Planescape Dungeons and Dragons setting. Attended to by enigmatic keepers without obvious motives, Sigil is considered the hub of the multiverse and is a vast melting pot for all cultures. Additionally, its strange, ring-shaped geometry (divided into Wards) means other parts of the city are constantly visible overhead: something it has in common with both the Presidium and the Wards of the Citadel.
 * The view from within the Presidium is reminiscent of the interior of the eponymous spacecraft from Rendezvous with Rama by Arthur C. Clarke.
 * Galactic Standard Time strongly resembles Decimal Time, a representation of time of day briefly utilised in Revolutionary France. The only major difference between the two is that one GSD (Galactic Standard Day) includes twenty hours, whilst one day in Decimal Time only has ten. Thus a second in Decimal Time is only approximately 1.16 times as long as a Terran Coordinated Universal second.
 * According to Mass Effect: Incursion, in 2183 there were 2,491,356 humans living on the Citadel.
 * In the PC version Mass Effect 3 Demo there is a video (ME3_ProCit_Loop.bik) that shows the Citadel from back. http://img515.imageshack.us/img515/976/me3democit.png