The assembly hall of the Lycian League. Overlooking the theatre in the south and the State Agora in the east, the structure constructed on a foundation sitting on the bedrock measures 421,8 x 30,6 m. It has a semi-circular wall forming the western edge of the structure. Preserved up to the roof level in the northwestern corner, the bu9ilding was constructed using local limestone blocks in the shape of a small theatre. In the centre, there is a small orchestra with marble flooring and it has a semicircular cavea consisting of 21 rows of seats with a seating capacity of up to 1400 people. In the central section of the cavea, there is a seating arrangement reserved for Lyciarchs or Governors, which is defined as tribunalia.
The assembly hall (bouleuterion) has several architectural periods. The structure was first constructed in the Late Hellenistic Period (early 1st century BC) as the assembly hall of the Lycian League, which was formally established in 176, the city of Patrai being the capital. The alterations in the second period may be associated with the annexation of Lycia to the Roman Empire as a Roman province, where Patrai maintained its status as the capital city of Lycia. In this period, most probably during the reign of the Roman Emperor Claudius (43-51 AD) or the Emperor Nero (MS 51-69) the cavea was extended in size and the semicircular wall in the west was connected to the northern and southern walls. In the third period, after a major earthquake hit the Lycian cities in 142/143 AD, a stoa was added outside and a stage building inside. The relevant functional modifications include the addition of a secondary function where the structure was also used as a concert hall (Odeion) along with its assembly hall function. In the fourth period, most probably in the 5th century AD, the city was reduced in size through the construction of a strong fortification wall with the stone blocks and material from the structures around and this Bouleuterion/Odeion structure, after serving about 600 years, was incorporated into the fortification wall as a bastion.
In his masterpiece “The spirit of the laws” (1748), the famous French thinker proposed Lycia as a\ model of an excellent confederate republic, and [that] this monumental edifice once housed the “history’s earliest and perfect example of government. The Grand National Assembly of Turkey has undertaken the restoration and preservation of this monumental edifice, which is truly special with its design and function.

Source: Notice at the site.