The Citadel

The Citadel is the headquarters of the Tri-Council, an alliance between three races of the Galaxy (Asari, Salarians and Turians) which has existed for millenia. Since its founding (and the discovery of the Citadel) many other races have joined the Tri-Council, from Humans to Quarians. The purpose of the Tri-Council is to monitor and protect Mass Relays, which are ancient, giant structures built by an ancient race that are used to blast ships through space, faster than warp speed, hyperspace and slipspace combined.

The awesome power of the Mass Relays means that guarding them and making sure they don't fall into the wrong hands is a necesseity. The Citadel itself is the base of operations of the Tri-Council, and each member race has an embassy here. Since only races that are members of the Tri-Council are legally allowed to use the Mass Relays, thousands of races have applied for membership and the Citadel is often teeming with life.

The Citadel is like a world in itself, it has shops, zoos, parks and random staircases that lead nowhere. The Citadel even has its own 'native' race, the Keepers, who were built by whoever built the Citadel in the first place (assumably the same people who built the Mass Relays).

Unknown to the Tri-Council, the Citadel is in fact a massive Mass Relay itself, which was revealed during the Reaper War. The Reapers theorised that, if they were to take over the Citadel and therefore the Mass Relays, they would have in their control a power that would make them truly unstoppable. It is for this reason that the Citadel came under heavy attack by the Reapers during the war, and the Reapers used its ability as a giant Mass Relay to propel themselves across the Galaxy.

The Citadel was critically damaged during the war but it is now in a stable state. However, its function as a Mass Relay was disabled during the fighting, and the Keepers seem to be unable to repair it. The Citadel is therefore one of the few Mass Relays to ever be destroyed.