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The Citadel is an ancient deep-space station, presumably constructed by the Protheans. Since the Prothean extinction, the Asari came to re-discover the station which numerous species have come to call home. It serves as the political, cultural, and financial capital of the galactic community. To represent their interests, most species maintain embassies on the Presidium, the Citadel's inner ring.

The Citadel Tower, in the center of the Presidium, holds the Citadel Council chambers. Council affairs often have far-reaching effects on the rest of the galactic community. Five arms, known as the wards, extend from the Presidium. Their inner surfaces have been built into cities, populated by millions of inhabitants from across the galaxy.

The Citadel is virtually indestructible. If attacked, the station can close its arms to form a solid, impregnable shell. For as long as the station has existed, an enigmatic race called the keepers has maintained it. Despite this, however; it still managed to suffer significant damage during the Reaper Wars about 400 years ago and even remained decommissioned above the planet Earth for quite some time. Only trough the galaxy's combined efforts was it able to be fully repaired and guided back into the Serpent Nebula.

The Citadel was only reported to be fully repaired 100 years ago, 300 years after the end of the Reaper War, though it had been running normally sometime before that.