To date, the theatre at Thoricus is the oldest known theatre in existence. Located north of Lavrion, almost adjacent to the city, it dates to the end of the Archaic era, between 525 and 480 B.C. But that is not its only distinction. Unlike Greek theatres built in later eras, it is elliptical rather than circular in shape, and has a rectangular, rather than circular orchestra. With 21 rows of seats, the theatre had a seating capacity of 4,000 people. On the east side of the orchestra, sculpted out of the natural rock, is the base of the ancient temple, and a room, complete with benches, also sculpted from the rock.
Dating from the mid-5th Century, the temple was used for meetings of the Demos’ authorities, and, like the small temple and room complex of Dionysus, it also served the theatre when it was in operation; It had a wooden scene, which, unlike later theatres, was never replaced with one made of stone. The area of the theatre was never intended solely for theatrical performances, but was also used for meetings of the citizens of Thoricus.
Source: www.athensattica.gr