Mass Effect & Mass Effect 2
The Ring is an enclosed loop of park-like space serving as the connection point for the Wards. The interior walls are lined with the embassies of influential species and private residences for the galaxy's elite.
The Presidium is full of 'open-air' restaurants, bars, and luxurious meeting areas. Gravity is about 1/3 Earth-normal. A holographic 'sky' is projected over the 'ceiling' of the ring. Unlike the 24/7 bustle of the Wards, the Presidium maintains a 20-hour day schedule, with a six-hour 'night' where lights are dimmed and the sky goes through a night cycle.
Offices and residences are often open to the interior. It is not unusual for embassies to have no exterior wall at all. This does not cause a crime problem due to the heavy C-Sec presence and ubiquitous monitoring devices on the Presidium. Thieves are quickly identified and apprehended.
The Ring is the location of the Citadel's spaceports. Being closer to the center of spin, there is less motion for a ship to match, and the reduced spin gravity makes handling cargo easier. Hundreds of ships pass through the Citadel every day, and every species with an embassy is granted a private dock.
The Tower, at the center of the Ring, holds the administration of the Citadel Council. The Tower rises over a kilometer from the ring, appearing to thrust forward parallel to the ward arms. As the tower is at the center of the spin axis, it experiences little centrifugal force. Gravity is maintained using mass effect fields at a 90-degree angle to the Ring and Wards.
A Consular dock can be found at the base of the tower. While normally used for diplomatic couriers and Spectre business, the shuttles docked here can evacuate the Council government in an emergency.