Mass Effect: After Armageddon
A massive space station located in the Serpent Nebula, the Citadel is the center of galactic politics and self-appointed capitol of the Milky Way. The Citadel Council governs the station and all recognized civilizations in Council Space. Several governments without the influence to have a representative on the Council itself have embassies on the Citadel.
Citadel security consists of several branches: C-Sec polices the station itself, C-Sec Wings is a naval offshoot of C-Sec which patrols Council Space, and the Spectres are elite, independent operatives with extrajudicial authority who report directly to the Council. Due to widespread pirate activity on the frontiers of Council Space, private security companies are also flourishing.
The Citadel can be divided into two main sections, the Presidium and the Wards. The wards are further divided into five areas — one for each of the five "petals" of the Citadel. The Presidium comprises the stations central ring.
The wards are:
The asari were the first species to find the Citadel, followed by the salarians several decades later. The two species cofounded the Citadel Council and established the seat of galactic government on the Citadel. As a reward for their role in suppressing the Krogan Rebellions, the turians were the next to get a Council seat, despite the fact that several other species had been a part of galactic civilization for much longer. Humanity was granted the fourth Council seat after Commander Shepard's efforts saved the Citadel from a reaper attack in 2183.
Shepard's investigations into the reapers revealed that the Citadel and mass relays, previously assumed to be relics of the extinct prothean civilization, were actually constructed long before by the reapers to aid in culling organic life every 50,000 years.
Prior to the Reaper War, few understood the Citadel's inner workings because it was maintained by silent and docile creatures who came to be known as the keepers. The keepers have not been seen since the Battle of Earth ended the war and deactivated the mass relays.