During the construction of the big temple of Apollo a platform was made, sustained by two large supporting walls. The northern one served to protect the temple from eventual fall of rocks. The southern aimed at retaining the soil around the foundations of the temple and define the area of Halos. It is this polygonal wall, just behind the Stoa (gallery) of the Athenians that the visitor sees, while walking uphill on the Sacred Road. The wall had been constructed in the second half of the 6th century, after 548 b.C., when fire had already collapsed the older temple. Later on, in the 5th century the Gallery of the Athenians had leaned against this wall; today, some traces of the gallery are still visible on it. The polygonal wall was constructed on the place of several older buildings of the Archaic period, among them as the well known vaulted building; the buildings were demolished, and the area was filled with earth and flattened.

The construction follows the Lesbian-style masonry: irregular blocks with curved joints are interlocked to fit exactly each other. The floor plan of the wall is Π-shaped with a length of 90 meters at the main face. The upper part, now missing, was built in isodomic masonry; there were some four or five rows more, thus the wall was originally some 2 meters taller than it is today. The face of the wall is inscribed with more than 800 inscriptions; most of them are resolutions related to the emancipation of slaves and are dated in the 3rd and 2nd centuries b.C.