Pnyx, from the Observatory

As a rule, the Ekklesia met at its own special meeting place known as the Pnyx, a large theater-shaped area set into the long ridge west of the Acropolis. In theory every assembly represented the collective will of all the male citizens of Athens, although the actual capacity of the Pnyx never seems to have exceeded 13,500, and for much of the Classical period it held only about 6,000. (American School of Classical Studies at Athens)