Hadrian's Arch was the entrance to the ancient city of Athens, built by(or for) the emperor Hadrian around 132 AD. The side of the arch that you see as you look into the city reads "This is the City of Theseus" while the other side reads "This is the City of Hadrian, not of Theseus". From the arch you look up Lissikratous Street towards the monument of the same name. Of course those are the walls of the Acropolis above the Plaka.