http://agathe.gr/guide/middle_stoa.html
Middle Stoa The appearance of the south side of the Agora was radically changed during the 2nd century B.C. with the construction of several new buildings. This South Square, as it is called, was made ... First to be built was the Middle Stoa, which ran east–west across the old square, dividing it into two unequal halves and obscuring one of the original boundary stones at its western end. At just under 150 meters long, it is the largest building in the Agora, with colonnades at both north and south as well as down the middle. Traces of a narrow parapet that ran between some of the columns can be made out on individual drums. The original steps and three columns remain in situ at its eastern end; to the west, only the heavy foundations of reddish conglomerate survive.