A-ring

The  is a small sector within the Citadel. It can be reached directly from the courtyard, the entrance is on the right when you enter the Citadel.

The A-Ring contains Lyon's Den, the Great Hall, the Library and entrances to B-Ring as well as Citadel Lab. The Lyon's Den is the quarters of Lyons' Pride while the Great Hall serves as a briefing room for high-ranking Brotherhood members and the Pride. Their meetings can be overheard occasionally, they don't mind if you walk in either.

The Library contains the Vault-Tec terminal you need to consult as part of Picking up the Trail. Another terminal reveals some background information on the Maxson family and various tidbits of information about the history of the Brotherhood.

Unmarked quest: Collecting Holotags
Scribe Jameson in the Library pays for any Brotherhood Holotags you bring her. The tags can be found on dead Brotherhood of Steel members all over the Capital Wasteland, primarily in the ruins of Washington, DC.

Unmarked quest: Gallows Humor
The unmarked quest Gallows Humor can be started here by talking to Paladin Kodiak. With a successful speech check, you can find out Knight Captain Gallows' name and get 1,000 caps as a reward.