Citadel

Supposedly constructed by the long-extinct Protheans, this colossal deep-space station serves as the capital of theCitadel Council. Gravity is simulated through rotation, and is a comfortable 1.02 standard G's on the Wards and a light 0.3 standard G's on the Presidium Ring.

The asari were the first race to discover the Citadel, soon after learning to use themass 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.

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 thekeepers 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 todark 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. They succeeded, but it was already too late for the Protheans. Because the Conduit portal only linked in one direction, Vigil feared that the Prothean scientists, unable to find any food or water on the Citadel, slowly starved to death. Due to this Prothean intervention, the Reapers were forced to travel to the Milky Way without the help of the mass relays, at normal FTL speeds.

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.

At the height of the Reaper invasion in 2186, the races of the galaxy unite to construct the Crucible, a superweapon of ancient design with the power to destroy the Reapers once and for all. The final and most important component of the Crucible is the Catalyst, which is revealed to be part of the Citadel. Unfortunately, the Illusive Man learns of this and flees to the Citadel, where he alerts the Reapers of the galaxy's plans. The Reapers respond by seizing control of the Citadel and moving it into Earth's orbit to protect it. The station's arms are closed, sealing it off, thereby preventing the Crucible from docking. The Alliance leaders determine that the only way to get the Citadel's arms to open is to get someone inside the Citadel and manually open the arms.

During the battle for Earth, Commander Shepard boards the Citadel via a transport beam and opens the Wards to allow the Crucible to attach to the station. Shortly afterward, Shepard meets the Catalyst - now revealed to be the Intelligence that controls the Reapers. Shepard must then choose between destroying the Reapers, controlling them, or merging organic and synthetic life. Upon deciding, the Crucible releases its energy and the Citadel, with its Wards now open to their maximum extent, amplifies it and transmits it to the Charon Relay, which in turn transmits it to the rest of the Mass Relay network.