Although this marble gateway looks like a gate, do not be mistaken, it was never a part of any wall. It is a commemorative monument, erected in honor of the Roman emperor Hadrian - a great admirer of Greek culture and Athens in particular, who helped build an entirely new quarter of the city.
At the time of its construction in 131 AD, it used to stand on the pathway between the old Acropolis and this newly-built part of Athens, perhaps to form a boundary between the two. Notice the Latin inscriptions on each side of the arch. The first one, facing Acropolis, says "this is the city of Theseus" while the other, turned towards the area constructed under Hadrian's reign, reads "this is the city of Hadrian, not Theseus."
Made of Pentelic marble, the arch is still well-preserved today and is popular among visitors as a great photo shoot location.
Address Leoforos Vasilissis Amalias 38 Athens 105 57, Greece
Coordinates 37°58'12.672" N 23°43'55.247" E