Mass Effect & Mass Effect 2
The majority of the Citadel's population lives in the Wards, the five massive arms of the station that house the residential and commercial districts. Many galactic races have established cultural enclaves here. Population density and cost of living are extremely high, akin to Earth cities such as Hong Kong and Singapore.
The Wards are open-topped, with skyscrapers rising from the superstructure. Towers are sealed against vacuum, as the breathable atmosphere envelope is only maintained to a height of about seven meters. The atmosphere is contained by the centrifugal force of rotation and a 'membrane' of dense, colorless sulphur hexafluoride gas, held in place by carefully managed mass effect fields.
The view from the Wards is spectacular. In the background, stars, Serpent Nebula, and the nearby blue giant called 'the Widow' move across the sky as the station rotates to stabliize itself. In the foreground, the lights of buildings and vehicles on the opposing Ward arms perpetually shine. The Citadel has no real day or night. While the station keeps to standard galactic time for political functions, businesses rarely close, and residents acclimate to sleep and work according to personal need rather than a day-night cycle.
Additions and modifications are constantly being constructed, though they must stay within certain specifications that will not compromise the operation of the station. Occasionally, the keepers will descend on an area of the Wards and move or change the architecture without explanation. Residents have learned to live with these inexplicable intrusions.