The Citadel is the closest thing to a university that exists in Westeros. A center of knowledge that has emerged organically over millenia, it is named for the fortress in which it is located. At the heart of Oldtown lies an ancient structure said to once be the stronghold of a pirate king, older than the First Men by thousands of years. As civilization has expanded around it, those bearing and seeking knowledge have both journeyed to the Citadel to teach and learn. A sort of barter system for knowledge exists in the strange institution, where those seeking to study must needs either pay to help the costs of upkeep on the massive fortress, or else teach another field in return. In this way, a wide variety of information has been exchanged over the centuries.
It should be no surprise that the Citadel boasts the largest library in Westeros, with works ranging from contemporary bound books all the way back to rune-etched stones and skins dating from early encounters between the First Men and the Children of the Forest.