Hadrian's Library, built by the emperor in 132 CE, was the largest public building of Roman Athens — not just a library but a complex of lecture halls, reading rooms, and an inner courtyard with a long reflecting pool. The papyrus scrolls were kept in alcoves on the east wall; the small windows you can see today were once filled with marble screens.
The most striking surviving feature is the west wall, with its row of Corinthian columns standing proud above the modern street. The site lies between Monastiraki and the Roman Agora and can be added to the same circuit for a small extra fee. A 5th-century Byzantine church was later built over the central courtyard, and its remains now form a kind of building-inside-a-building.