The Citadel is an ancient deep-space station, presumably constructed by the Protheans. Since the Prothean extinction, numerous species have come to call the Citadel 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.