Location from Mass Effect by BioWare
The capital of galactic civilization — a massive Prothean station that is also, secretly, the key to the Reaper invasion plan.
The Citadel is both the seat of galactic government and the most prestigious address in the Milky Way. The Presidium — the inner ring — houses the Council chambers, embassies, and high-end residences. The Wards — the five arms — are the residential and commercial zones where most of the population lives, works, and argues. C-Sec, the Citadel Security force, maintains order with varying degrees of success. Every major species has an embassy. Every deal of consequence gets made here eventually. The Normandy docks at Zakera Ward or the Presidium depending on which elevator you take. The Citadel was supposedly built by the Protheans. It wasn't. The Reapers built it. The mass relay at its center is the one that would bring the Reaper fleet through dark space when activated. This is Sovereign's play in ME1. The station's keepers — small bio-mechanical creatures — maintain all the station's systems without complaint or apparent understanding. They were programmed. Understanding is not part of their function.
A massive space station in the Serpent Nebula shaped like a pentagram — a central ring 7.2 kilometers across with five arms projecting outward, each 43.6 kilometers long. The arms can close into a cylinder in emergencies. When open and lit, the station is visible from enormous distances and unmistakably artificial at any scale. Population 13.2 million, not counting the Keepers who maintain the station's systems.
Also known as: Citadel, The Citadel, The Station