Altar of the Twelve Gods
Near the middle of the open square, somewhat to the north, lay the Altar of the Twelve Gods (Fig. 7), today largely hidden under the Athens–Piraeus railway (1891). A corner of the enclosure wall survives, along with the inscribed marble base for a bronze statue that reads "Leagros, the son of Glaukon, dedicated this to the twelve gods." Thucydides tells us the younger Peisistratos, grandson of the tyrant, established the altar in the Agora during his archonship (522/1 B.C.). The upper surface of the present sill (4th century B.C.) preserves traces of the stone fence that would have defined the sacred area around the altar, now missing.
The altar was one of the few monuments permitted within the open square and it served as the zero milestone or center of the city. Herodotos (2.7), when giving a distance in Egypt, tells us that it is as far from Heliopolis to the sea as it is from the Altar of the Twelve Gods in Athens to Olympia. On a milestone dating to ca. 400 B.C. we read: "The city set me up, a truthful monument to show all mortals the measure of their journeying: the distance to the altar of the twelve gods from the harbor is forty-five stades" (IG II2 2640). Physically, we are at the heart of the city.
"Amongst those of the Peisistratids who held the annual magistracy at Athens was Peisistratos, son of Hippias the tyrant (named after his grandfather), who during his archonship set up the Altar of the Twelve Gods in the Agora and the Altar of Apollo in the shrine of Apollo Pythios. On the altar in the Agora the Athenians later rendered the inscription invisible by adding to the length of the structure." (Thucydides 6.54.6–7)