Citadel

The Citadel Station is a massive space station, constructed millennia ago in the heart of the Serpent Nebula near the star known as the Widow. Heavily populated with diplomats and traders, it serves as the home of the Citadel Council, a powerful multi-species governing body. The Citadel is believed to be the greatest achievement of the Protheans.

Statistics

 * Total length (open): 44.7km
 * Diameter (open): 12.8km
 * Thickness of Ward arms: 330m
 * Presidium diameter: 7.2km
 * Exterior armour thickness: 13m
 * Gross weight: 26.4 billion metric tons
 * Population: 13.2 million

(Source: Galactic Codex: Essentials Edition 2183)

History
The asari were the first race to discover the Citadel soon after learning to use the mass relays. They were joined shortly after by the salarians, then the volus. As other races were contacted by the asari or discovered the Citadel in turn, they gained embassies on the Citadel and came under the jurisdiction of the growing Council, which eventually became 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 nexus 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, the Citadel still has many secrets. No one knows exactly how old the station is nor what resilient material it is built from. The location of the Citadel's core and its master control unit, regulating systems like life support and navigation, is still unknown. But the most interesting mystery of the Citadel surrounds its caretakers, the keepers. These mute alien creatures maintain the Citadel's systems and can be seen working all over the station - often rearranging things - but nothing is really 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 done for defensive purposes, however, and normally the station retains its star-shaped layout.

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. The central ring is also home to the Citadel's docking bays; it is the centre of rotation and easier for ships to match their course and speed to. The Presidium is regulated by a VI named Avina.

Citadel Tower
The tower at the centre of the Presidium houses the Council Chambers and the main traffic control for the station - any ship within a few thousand kilometres 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 appointed 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 like Hong Kong or Singapore. Barla Von calls the Wards the cultural heart of the Citadel and therefore of Citadel space.

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

The next 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. Should any aggressor manage to break through the Citadel Fleet, reinforcements from all over the galaxy are only minutes away, thanks to the network of mass relays located nearby, as are reinforcements from allied and friendly fleets, such as the Systems Alliance Navy.

If any enemy gets close enough, the station itself, as previously noted, 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 city skyline of the Citadel seen in the Presidium is clearly inspired by the skyline of a future Los Angeles in Blade Runner.

The Citadel was not, in fact, constructed by the Protheans; they were simply the last race before the asari to make use of it. It, and the relays, are the creations of the Reapers, built to help facilitate their repeating 50,000 year cycle of genocide. The Reapers were also the ones who placed 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 at a stroke.