Next to one of the busiest avenues of modern Athens, the arch (or gate) of Hadrian was built during the reign of Roman Emperor Hadrian, probably in 130 or 131 C.E.
Unlike the typical Roman arches, which tend to be bulky and sturdy (like this), this one is slender and light. It is if the architect went out of his way to make this arch different from the others, or perhaps to make it fit the other monuments of the city.
Standing under it, one can see the Acropolis on one side, and the Temple of Zeus on the other. In this photo, taken from the West, the columns of the latter are just visible under the trees behind the arch.