A large rectangular building located on the north-eastern side of
the ancient Agora. It was built a short distance from the main area of the Agora, on the
other bank of the river Elisson.
It was used as the meeting place of the "myriad", the 10.000 members of the Arcadic confederation. According to Pausanias, the foundation of the building was attributed to Thersilos, around 4th century BC. The building was constructed after the foundation of Megalopolis by Epameinondas, in 371 BC, after the defeat of the Lacaedemonians by the Thebeans at Leuctra.
At the south side of the auditorium was a rectangular "orchestra". Along the other three sides there were rows of wooden seats. The 65 columns supporting the roof, were arranged in lines radiating from the center, to ensure visual contact with the speaker.