This remarkable polygonal wall supports the platform on which stands the Temple of Apollo, and defines the area of the Halos, or threshing floor, to the north-west. It was raised in the second half of the sixth century BC, probably after the destruction of the first temple in 548 BC and before the construction of the Alkmaionides temple in 513-505 BC. The fifth century BC Stoa of the Athenians was built against this wall, and traces of it are visible on the wall's surface. Prior to the construction of the wall, the area was leveled and several early Archa?c buildings and treasuries, including the famous apsidal structure, were destroyed or buried under fill.

The wall is built in polygonal Lesbian-style masonry, with irregular interlocking blocks with curved joints. In plan it is Pi-shaped, its main face ninety metres long. The upper four or five courses consisted of isodomic masonry, now missing; the wall was originally approximately two metres taller that it is today. Its dressed face was covered in the third and second centuries BC with more than eight hundred inscriptions related above all to the emancipation of slaves.

The wall was conserved in recent years.