Hadrian’s Library is located near Monastiraki Square. It can easily be seen from the outside, but if you are curious (like me), you can enter to learn much more about the site, get close up to the foundations of some early Christian churches, and visit the small museum to see some finds from the site. Entry to the site is €4 as of July 2016, but a €30 combo ticket allows access to several archaeological sites in Athens, including the Acropolis.
The library was built by Roman Emperor Hadrian in 132 AD, destroyed in 267 AD by the Herulians, and rebuilt by the prefect Herculius in 412 AD. The main east room of the library once held 17,000 books. There was also a small amphitheatre on the site.
During Byzantine times, three churches were built on the site. They were constructed in the 5th century, 7th century, and 12th century. The 12th century church was the first cathedral of Athens, Megali Panagia.
In the small museum, a statue of Nike is on display among other artifacts excavated at the site.