A rectangular building (122 x 82 m) built in 132 AD by the Roman emperor Hadrian. Seen from above, it comprised a peristyle courtyard (with a colonnade), with special rooms for keeping papyri and books, lecture halls, and so on. The impressive structure was partly destroyed by the Heruli in 267 AD, and repaired at the beginning of the 5th century AD. At the same time, a beautiful quatrefoil church was constructed in the grounds of the atrium; a luxurious structure with exquisite mosaics, thought to be the work of the empress Athenais-Eudocia.
Sometime between the 11th and 12th centuries, a small church was built on the west propylaeum of the Library, where Areos St is today. This was known as, the church of "Aghioi Asomatoi on the steps" (it is now demolished). Also in the 11th century, the early Christian church in the center of the atrium was transformed into the medieval church of Megali Panaghia.
During Ottoman rule, this was the location of the Upper Bazaar, the commercial center of the period, with more or less a hundred proprietors. On the southwest side was the Voevodalik, the residence of the Voevod (the Turkish Commander). Close by, near today's Mitropoleos St, was the seat of the Demogerontia, or Kouseyio, the seat of the Greek Administration. Until the end of Ottoman rule (1833), this area served a purpose similar to that of the ancient Greek Agora - an administrative and commercial center - for the Athenians. The bazaar at the east end of the Library was burnt down in 1884. Excavation and study of the monument then commenced, and it opened to the public for the first time in the summer of 2004.