In series lore, the Citadel is one of the largest artificial structures in the galaxy, with a population of 13.2 million intelligent beings from across the Milky Way galaxy, and uses
centrifugal force to create
artificial gravity for its inhabitants. Initially, it was discovered by the
asari and the
salarians, the earliest post-Prothean races to discover the mass relays - megastructures scattered throughout the galaxy that facilitate
FTL travel. Following this, an executive committee known as the Citadel Council was created, with the station functioning as the seat of galactic government. The Council holds great sway in the galaxy, and are recognized as an authority by most of explored space. The Citadel is capable of transforming from an open space station into an impenetrable shell, with each of the five sides containing a "ward" region about the size of Manhattan, while the middle ring contains a hub area known as the Presidium. The Citadel's denizens are instructed never to interfere with their maintenance work within the inner systems of the Citadel.
Citadel society VI hologram integrated into various Citadel information terminals Three politically dominant species make up the Citadel Council at the inception of the series: the asari, a
feminine species who favor
centrism,
republicanism, diplomacy, biotic abilities and
guerrilla warfare tactics over raw military power; the salarians, an amphibious species who have an aptitude for
liberalism,
technocracy, technological research, espionage and
unconventional warfare; and the
turians, a proud avian-like species known for their
conservatism,
stratocracy, disciplined
honour culture and
military-industrial capability. Membership of the Citadel Council is eventually expanded to include a human representative by the end of the first
Mass Effect. A few alien races are depicted as formerly part of the Citadel space community during the series: the
krogan, a hardy and warlike reptilian species in decline in the aftermath of the Krogan Rebellions against Council authority; the batarians, an isolationist people who recalled their embassy staff from the Citadel in protest against the integration of humanity into interstellar society; and the
quarians, a race of
immunodeficient nomads who were forced to flee their homeworld Rannoch on a migratory fleet of ships when their development of
advanced networked synthetic intelligence created the
sapient geth, who warred with their creators. == Development ==