Agora, Twelve Gods, Agora Short Guide, Agora Guide 4

Altar of the Twelve Gods. Originally constructed 522/1 B.C., but with later rebuilding and repairs. View from the southwest. Only the southwestern corner of the altar is visible (the rest of the monument is actually underneath the electric railway to the north). What we see in the photos is only a low sill with cuttings and holes to attach a slabs and posts of a stone fence that once surrounded the altar. The original entrance to the altar (now hidden by the railway wall) was just to the left of the marble statue base (far left in the photo).
The Altar of the Twelve Gods was considered to be the very center of the ancient city of Athens, and it was the "zero mile marker" from which distances were measured.