Citadel

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

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

See more: Codex:Citadel Statistics

History
The asari were the first council race to discover the Citadel, soon after learning to use the mass relays. They were joined shortly by the salarians, and thereafter 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, which 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 name given to all systems that come under the Council's authority.

Even after thousands of years of occupation, the Citadel remains an enigma. 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 shaped like a five-pointed star. It consists of a central ring 10 kilometers in diameter, ringed by five arms, each of which is 25 km long and 5 km in breadth. The station is capable of closing in on itself, transforming into a long, cylindrical tube as the five arms come together. This is only practiced for defensive purposes, however, and normally the station retains its star-shaped layout. The central ring is home to the Citadel's docking bays; it is the center of rotation, facilitating easier access for ships attempting to dock with the station.

Presidium
The central ring itself contains the Presidium, 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 is also a shop and offices for various services. The Presidium is monitored by a VI named Avina.

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 are known as the Wards and are, effectively, individual cities housing millions of people. The towers are sealed against vacuum because the atmosphere is only maintained to a certain height. The population density of the Wards is comparable to Earth cities such as Hong Kong or Singapore. Barla Von calls the Wards 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.

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 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 strong enough that, even using the most advanced weaponry available, it would take several days of sustained bombardment to inflict any serious damage to the station.

In addition, 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.

Inspiration

 * The Presidium recalls the centrifuge from Discovery One, the starship in 2001: A Space Odyssey, but in fact it almost exactly resembles certain artists' visions of future space-colony, the Stanford Torus, that 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.
 * The city skyline of the Citadel seen in the Presidium may be inspired by the skyline of a future Los Angeles in Blade Runner. The cities seen on the other arms take inspiration from the cities of the planet Coruscant in the Star Wars universe.

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 50,000-year cycle of galactic genocide. The Reapers were also responsible for the placement of the keepers on the station, so that the occupying races would not discover the Citadel's central functions. The station is in fact 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 one stroke.

According to Vigil, the last Protheans used the Conduit and travelled to the Citadel to try and break this cycle. However, the Conduit portal only links 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 Protheans' warning that the station is a 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.