The Citadel

The Citadel is the heavily defended headquarters of the Capital Wasteland Brotherhood of Steel, built into and beneath the ruins of the Pentagon in Washington, DC. It is located on the west end of Arlington Memorial Bridge, northeast of Arlington Library, and initially consists of 4 interior sections.

Background
The ruling council of the Brotherhood of Steel, based in the Lost Hills bunker in South California, decided to send a contingent of soldiers to the East Coast, to recover any and all advanced technology from Washington, DC and to investigate the reports of super mutant activity in the area.

The Pentagon, as they had feared, was largely destroyed. But its sub-levels remained intact, and contained enough pre-war technology and weaponry to keep Brotherhood forces going indefinitely (once the defense robots were destroyed). But there was something else, as well… They found Liberty Prime, a technological marvel that, if restored, could help the Brotherhood rebuild a strength and reputation that had been declining steadily for years.

The discovery was significant enough to earn Paladin Owyn Lyons a battlefield promotion to Elder, and a new directive from his superiors – to establish a new, permanent Brotherhood base in the Capital Wasteland, and continue the search for any other advanced technology hidden in the capital’s ruins.

Lyons accepted his new post gladly, and founded the Citadel, built into and beneath the ruins of the Pentagon. It was a fortress the Brotherhood of Steel desperately needed, and one they rushed to fortify, thanks to their other great discovery – Super Mutants.

Exterior
The Citadel is a large fortress with walls of stone and steel. It retains its shape which gave it its name the "Pentagon" and has only a single entrance, a large metal gate usually guarded by multiple paladins. A small corridor leads from the main gate to a door which provides access to the inner bailey.

The inside is inaccessible until you complete The Waters of Life segment of the main quest. Entry is scripted as part of the completion of this quest. Before then, the guard commander Paladin Bael will gruffly deny your entry.

Bailey
The fairly large bailey serves as a training and sparring area. Under the command of Paladin Gunny, new recruits practice various skills from unarmed combat to firearm usage. To this end, the bailey contains shooting ranges and other installations for combat practice. A large hatch in the middle of the courtyard connects to the Laboratory, although it cannot be used as an entrance by the player. The Laboratory and A Ring, sections of the buildings which surround the yard, can be accessed through doors opposite and next to the main entrance.

Citadel - A Ring
The A Ring is a small sector within the Citadel which can be accessed directly from the bailey. 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 Laboratory. 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.

If you have high Karma level you can take Paladin Cross with you.

In the Library you can find 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.

Citadel - B Ring
The B Ring is another section within the Citadel which can only be reached from the A Ring via two doors in the back. It consists mostly of sleeping quarters for all Brotherhood of Steel members as well as a medical bay and the private quarters of Owyn Lyons and Arthur Maxson.

Squire Maxson's room contains has a hackable terminal in which, among various children's stories and other personal entries, you can find the location of a Guns and Bullets magazine under the bed in the Lyon's Den (A Ring).

The medical bay is home to Sawbones, a Mr. Gutsy robot, who is the slightly malfunctioning doctor of the Citadel.

Citadel - Laboratory
The Laboratory is the third sector of the Citadel, connected to both the bailey and the A Ring. It spans two levels and consists of the main working area, the Armory and sleeping quarters for the Scribes. Various terminals with technical background information on weaponry are scattered all over the place.

The lab is naturally the working place for most of the Scribes save for those of the Order of the Quill. While they are conducting research on various fields such as threat assessment, weaponry and defensive technologies, their main focus is the work on Liberty Prime, a gargantuan pre-war robot and the reason the Citadel base was established in the first place.

Scribe Rothchild who is part of various main quests is usually to be found in the area just in front of the robot, on the bottom level. In the vicinity of the robot dock there's also a terminal with progress information on the work on Liberty Prime which lets you run a sound check.

On the wall at the bottom level there is a digital map display from which you can extrapolate the location of the Vault-Tec vaults in the Capital Wasteland if you have not already found them. After Scribe Rothchild has used it to show you the location of Vault 87, the map markers will automatically be added to your Pip-Boy 3000.

The Armory is located in an extra room on the bottom level and is run by Knight Captain Durga. She will only trade with you if you have Elder Lyons' permission which is easily obtained.

Citadel Armory
If you chose to take the option of destroying the Citadel in Broken Steel, you will find a door leading to the armory within the crater, and one on the far side of the circular path around the crater. This section is not to be confused with the regular Armory located in the Laboratory.

Within the armory, there are 4 First Aid Boxes and plentiful supplies of various ammos and weapons. In the offices beside the armory, there is a terminal describing several "incident reports" and a wall-safe (unlockable through the terminal) containing Callahan's Magnum and magnum

Notable loot

 * A Duck and Cover! book in a crate in one of the stalls in the bathroom of the Citadel Lab.
 * A Guns and Bullets book under the rightmost bed in A Ring's "The Den" area. Just outside this room, in the lounge area, there's a Brotherhood of Steel Holotag on a small curved table.
 * Smuggler's End, Elder Lyon's unique AEP7 Laser Pistol, can be found in his safe in B Ring's "The Solar" area. Occasionally it may be taken from the safe by NPCs.
 * 2 Tesla Cannons in the Citadel Lab (Broken Steel)
 * 2 Tesla Cannons in the Citadel Armory (Broken Steel)
 * Callahan's Magnum in a safe in the Citadel Armory (Broken Steel)
 * 3 Mini Nukes in the Citadel Armory (Broken Steel)
 * 72 Alien Power Cells in the Citadel Armory (Broken Steel)

Main quests

 * The Waters of Life
 * Picking up the Trail
 * Finding the Garden of Eden
 * The American Dream
 * Take it Back!
 * Death From Above (Broken Steel)
 * Shock Value (Broken Steel)
 * Who Dares Wins (Broken Steel)

Unmarked quests

 * Collecting Holotags
 * Gallows Humor
 * Worst. Doctor. Ever
 * Some Assembly Required (Broken Steel)
 * Mutant Blood Samples (Broken Steel)

Appearances
The Citadel appears only in Fallout 3.

Behind the scenes

 * "The Citadel" was the original name of the Cathedral in early design documents (like the Vault 13: A GURPS Post-Nuclear Adventure timeline).
 * The Citadel's name may be a nod to the Citadel Star Station from the game "Wasteland", which heavily inspired the first Fallout game.

Bugs

 * It is possible to enter the Citadel by glitching into a pile of rubble to the northwest of the Citadel (YouTube video). This is an effective way to unlock the Power Armor Training at an early point in the game, although various NPCs in the Citadel, when talked to, will tell you about events that might not have happened yet due to the non-completion of the main quest segment The Waters of Life. As seen in the video, leaving the Citadel after using this glitch will result in completion of the quest The Waters of Life, even if it was never begun or known about, along with all prior quests in the main quest arc that the player did not complete. It seems that using Fast Travel in the bailey avoids this glitch, which means that the main quest line can still be completed normally. (confirmed PS3 GOTY Edition, both "pile of rubble" glitch and Fast Travel avoidance afterwards)
 * There are at least two Brotherhood of Steel Initiates using the firing range, and one is firing a Laser Rifle. You will not be attacked if you shoot the Laser Rifle out of his hands, and he will start throwing Frag Grenades at the target. Many of his throws will either be off target or ricochet in various directions, causing numerous casualties among the nearby Brotherhood of Steel, and allowing you to search the bodies. (confirmed PS3/Xbox 360) When attempting this by using VATS, it is possible that if targeting the "Weapon" too many times, the Initiate will take out a grenade almost instantaneously after the rifle is shot. If this happens you may accidentaly shoot the grenade and kill the Initiate and anybody close.
 * If you fast-travel to the Citadel whilst an Enclave-controlled Deathclaw is following you, upon reaching the destination the Deathclaw will promptly die. From then on, whenever you fast travel to the Citadel, the same Deathclaw will constantly respawn and then die when you get there. This is a great way to collected an unlimited amount of Deathclaw Hands. (confirmed Xbox 360)
 * When you first arrive to the Citadel with Dr. Li and company, fast traveling away from the Citadel and fast traveling back before the gate opens will cause a bug in which Dr. Li and the rest of the scientists disappear and it will become almost impossible to enter the Citadel.
 * The Radio in the bailey will continue to play GNR as if Three Dog is still alive after the PC has killed him. Where as the Pipboy radio will play just music. (confirmed Xbox 360)
 * When you have finished the Broken Steel Add-On, there are no Paladins or Robots on the gate. (confirmed on all platforms)
 * There is an African American BoS initiate firing a laser rifle at the shooting range in the courtyard. If you pickpocket his frag grenade, leave the pickpocket screen, then pickpocket him again, he will have another frag grenade. This can be done an infinite number of times (all you need is to be successful every time, which can be achieved by frequent reloading of saves). So far this has only been confirmed on the X360.
 * If you make Fawkes hostile after firing him (not to shoot him, to let him go back to Underworld) near the Brotherhood of Steel, they will shoot Fawkes (except Vargas) as their mortal enemy Super Mutant. (confirmed on all platforms)