Built at the beginning of the 2nd century AD, the Selley Theatre could accommodate around 12,000 spectators, making it one of the largest buildings of its kind. It was modified in the Gallo-Roman age. Both the orchestra and the cavea (tiered seating) are more than semicircular, the rather modestly sized stage building was built onto the exterior of the linear façade wall, and the actual stage consists of a simple wooden platform which reaches into theorchestra.
In the latter part of the 3rd century i twas finally turned into a fortified retreat enclosed by a defensive ditch and was used as such until the mid-4th century.
References:The ancient Argos Theater was built in 320 BC. and is located in Argos, Greece against Larissa Hill. Nearby from this site is Agora, Roman Odeon, and the Baths of Argos. The theater is one of the largest architectural developments in Greece and was renovated in ca 120 AD.
The Hellenistic theater at Argos is cut into the hillside of the Larisa, with 90 steps up a steep incline, forming a narrow rectilinear cavea. Among the largest theaters in Greece, it held about 20,000 spectators and is divided by two landings into three horizontal sections. Staircases further divide the cavea into four cunei, corresponding to the tribes of Argos A high wall was erected to prevent unauthorized access into the theatron and may have helped the acoustics, but it is said the sound quality is still very good today.
Around 120 CE, both theaters were renovated in the Roman style.