The Ekklesiasterion (or Bouleuterion), fundamental monument of the agora, is a circular building housed the council of citizens, a basic institution of the city-state (polis) made up of the citizens' representatives who assembled in order to confer and decide about public affairs (voting the law or electing the magistrates).
The word bouleuterion is composed from Greek boule (council) and the suffix -terion (place for doing something).
Its construction is dated between 480 - 470 BC. It was a small circular council hall (bouleuterion) or assembly space (ekklesiasterion), with concentric treads, cut directly into the rocky bank and then covered with blocks. It was probably never roofed, but had a wall around it, perhaps with a small arcade round the inside. It is believed to have had a capacity of 1100 maximum 1700 people and was used for the political assemblies of the citizens of Posidonia when it was a Polis of Magna Graecia.
After the conquest of the Lucanians the structure retained its function while when the city became Roman the ekklesiasterion was covered with earth and a sanctuary was built at its top.