The Thersilion was the assembly hall (bouleuterion) of the Arcadian League, where the Myrioi?the 10,000 delegates of the Arcadian Federation?convened. It was named after its donor and dedicator, Thersilos of Orchomenos. It was among the first buildings constructed in the city, consisting of a rectangular hypostyle hall with a roof, measuring 66.65 x 52.43 meters. The interior columns were arranged radially, and the floor sloped gently toward the center to ensure clear sightlines to the speaker's dais from every point in the hall.