http://agathe.gr/democracy/the_ekklesia.html
The Ekklesia (Citizens' Assembly) All Athenian citizens had the right to attend and vote in the Ekklesia, a full popular assembly which met about every 10 days. All decrees (psephismata) were ratified ... In phase III (4th century B.C.) the structure was enlarged but retained the same general configuration. ... Throughout its long history the Pnyx had three major building phases. The earliest is generally associated with the Kleisthenic reforms. ... In a third and final phase dated to the late 4th century B.C., the seating capacity was greatly increased, to accommodate as many as 13,500 people.