Located northwest of Priene, on the northeast corner of Dilek peninsula, near today's Guzelcamli, stood Panionion, as an important annual meeting place of the Ionian League. Panionion is significant for its being the oldest unity of the colonies in the world. The league was a political union, and was consisted of the members from the "12 Ionian Colonies". Years later, the city of Smyrna (Izmir) joined the league.
The meeting in Panionion was usually after the harvest. The members were sharing their problems about business, art, farming and sea trade for improving their own cities or finding reasonable solutions to their problems.
Every year, in addition of the gathering of the members, a traditional festival was held to the honor of the God Poseidon of Helliconia. During the festival, young and strong men in Panionion were placing the cattles into the temple to be sacrificed, by hanging their horns. During the Hellenistic era, one day of the festival was decided to be celebrated to the honor of King of Pergamum, Eumenes II.
Serious excavations conducted by the German archeologist Th. Wiegard, have been revealed some very important artifacts from around the region. The remains of a circular wall on St. Ilias Crest are considered to be the meeting place of Panionion. On the south of St. Ilias Crest, stands a small church, named as "Hegeios Demetrios" by Th. Wiegand.
The church was once used for worshipping Poseidon, the God of Sea and Earthquakes. The higher parts of St. Ilias Crest was surrounded by the limestone walls, as to protect Panionion. A long thin structure stands in the middle, measuring 18x4, reflecting the architectural style of 6 BC.
The eastern and the southwestern parts of Panionion are not in good condition. Only, some sitting places and a Hellenistic turret -once used for observing and protecting the area, exist in the area. There is also a cistern to the further east of turret, including the clay-made pots in it, with the traces of the Hellenistic Age.