The triumphal arch lies on an ancient street (now
called Odos Adrianou) that led from the old city of Athens to the new,
Roman section, built by Roman Emperor Hadrian. It was constructed by the
Athenians in A.D. 131, in honor of their benefactor emperor. Two inscriptions
are carved on the architrave, one on each side: the first, on the side towards
the Acropolis reads "This is Athens, the ancient city of Theseus"; the second,
on the other side, facing the new city reads "This is the city of Hadrian and
not of Theseus". It combines a lower part based on a Roman arch with an upper
part based on the front of a Greek temple.
More information: http://www.athensinfoguide.com/wtsarch.htm, http://en.wikipedia.org/wiki/Arch_of_Hadrian