Although these marble doors look like gates, make no mistake, they were never part of the wall. This is a historical monument, erected in honor of the Roman emperor Hadrian - a great admirer of Greek culture and, in particular, Athens, who helped to build a completely new quarter of the city.
During the construction of the arch in 131 AD, it stood in the path between the old Acropolis and this newly built part of Athens, perhaps to form the boundary between them. Note the Latin inscriptions on each side of the arch. The first, addressed to the Acropolis, says: "This is Athens, the former city of Theseus," and the other, addressed to the area built under the domination of Hadrian, reads: "This is the city of Hadrian, not the city of Theseus."
The arch, made of pentaleum marble, is still well preserved and is popular with visitors as a great place to take photographs.