http://agathe.gr/guide/tholos.html
Tholos The south half of the west side was given over to the major administrative buildings used to run the Athenian democracy (Fig. 14). The buildings are poorly preserved, but the identifications are ... Built around 470 B.C., the building was an unadorned drum, with six interior columns supporting a conical roof of large diamond-shape terracotta roof tiles (Fig. 15).