An ancient station supposedly constructed by the long-dead Protheans, it serves as the hub of interstellar politics and society in the Milky Way galaxy due to its status as the capital of the Citadel Council, the dominant galactic polity within the Mass Effect universe.

[2] 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.

[5][6] Associate races who have embassies on the Citadel include the volus, a client race of the turians who always wear full-body environmental-suits outside of their homeworld; the hanar and the drell from the ocean world of Kahje, who associate with each other in a long-term, intimate symbiotic relationship called the Compact; and the elcor, a ponderous species resembling a cross between a manatee and a gorilla, who preface everything they say with the intended emotion due to their lack of vocal inflection and discernible facial expressions.

[5][6][7] 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.

Describing its concept as an expansive and highly livable setting, Hudson made an analogy to the New York City borough of Manhattan as a place populated with numerous cultures and where "thousands of stories" could be told.

Alex Meehan of GamesRadar+ called the Citadel "a realistic way of bringing all the different elements of Mass Effect's world under one roof", saying it was "chock-full of interesting people and exciting encounters".