The Citadel was founded many centuries ago to promote and increase learning and knowledge in Westeros. It is administered by the Conclave, the ruling council of archmaesters. The Citadel is also where young men from the entire continent are trained to become Maesters.
At Castle Black, Maester Aemon notes to Tyrion Lannister that the current summer has lasted nine years, and that reports from the Citadel say the days are growing shorter, indicating that its end is soon at hand. When Aemon later reveals his past to Jon Snow, the surprised Jon says "You're Aemon Targaryen!". However, Aemon corrects him by stating that he is "a Maester of the Citadel, bound in service to Castle Black and the Night's Watch", stressing to Jon that a sworn brother of the Night's Watch abandons all previous family ties.
A special white messenger-raven arrives at the Red Keep in King's Landing from the Citadel. Grand Maester Pycelle presents it before Queen Regent Cersei Lannister and the Small Council, explaining that it signifies that the conclave of the Citadel has met, considered reports from Maesters all over the Seven Kingdoms and officially declared the ten year long summer to have finally ended, and autumn has arrived in Westeros.
After the Fall of Winterfell, Theon Greyjoy convinces Maester Luwin to obey him by pointing out he is a Maester of the Citadel sworn to serve the Lord of Winterfell, a title that now belongs to him, as he has forced Bran Stark to claim publicly.
When Jaime Lannister is being treated by Qyburn at Harrenhal after losing his hand, he notes that Qyburn is not a maester, as he isn't wearing a maester's chain of office. Qyburn explains that the Citadel stripped him of his chain, expelling him from the Order of Maesters, because they found some of his experiments "too bold". Later, Roose Bolton points out to Jaime that by treating him and accompanying him back to King's Landing Qyburn hopes Tywin Lannister will reward him by forcing the Citadel to give him back his chain. During their journey to the capital, Jaime asks Qyburn why exactly did the Citadel take his chains, and Qyburn explains he experimented on living men, as "the only way to treat disease is to understand disease, and the only way to understand it is to study the afflicted."
In the trial by combat championed by Ser Gregor Clegane and Oberyn Martell, the former for the Crown and the latter for the accused Tyrion Lannister, Oberyn dies but he first poisons Clegane grievously with his spear, which was coated with Manticore venom. Cersei Lannister calls for Grand Maester Pycelle and Qyburn and, though the Grand Maester insists there is nothing to be done for him, Qyburn claims he can save Clegane. Pycelle protests, as Qyburn is not even a Maester, to which Qyburn replies that is for the best, as no maester knows how to save him. Pycelle exclaims that is the kind of arrogance that had him expelled from the Citadel, which considered Qyburn's experiments dangerous and unnatural, an opinion with which he agrees.
Samwell Tarly explains to Gilly that the Citadel, which harbors the largest library in the world, is located in Oldtown and it's where the maesters are trained. He then tells her about his wish to go to Citadel and train to become a Maester before he was sent to the Night’s Watch.
Maester Aemon dies of old age and Sam eulogizes him as a "maester of the Citadel, chained and sworn, and sworn brother of the Night's Watch, ever faithful."
After Jon returns from Hardhome, Sam tells him that he wants to go to Oldtown to train at the Citadel to be a maester, so he can return and take the deceased Aemon's place as maester of Castle Black. He also wants to take Gilly and her young son with him, explaining that if they stay here they will be among the first to die in the coming war against the White Walkers. Jon agrees and Sam gravely promises to return, before leaving Castle Black with Gilly and her son on a cart, beginning the long journey to the Citadel in Oldtown.
In the booksEdit
In the A Song of Ice and Fire novels, the Citadel is described as a university-like complex of buildings extending along both banks of the River Honeywine in Oldtown, linked by graceful bridges. The Isle of Ravens is the oldest part of the Citadel and the site of the ravenry, from where the maesters send messenger ravens out to all parts of the Seven Kingdoms. Other locations of note in the Citadel include the Seneschal's Court, Weeping Dock, and Scribe's Hearth.
Old Oak · Red Lake · Goldengrove · Bitterbridge · Tumbleton · Grassy Vale · Cider Hall · Ashford · Horn Hill · Brightwater Keep · Bandallon · Honeyholt · Oldtown (The Citadel · The Hightower) · Uplands · Blackcrown · Three Towers · Sun House · The Arbor