Fortress of Eleutherai, Archaeological site at Oinoi, Municipality of Mandra-Eidyllia, Greece.

The Fortress of Eleutherai is an ancient fortified site built on a hilltop, covering about 3.3 hectares with walls extending roughly 860 meters in length. Along the northern wall stand seven square towers, and the site includes a double main gateway facing west plus two smaller entrances on the north side.

Built between 370 and 360 BC, this fortress guarded the mountain pass between Attica and Boeotia as a crucial defensive position. It was one of several fortifications designed to protect the region from invasion.

Eleutherae was an independent polis on the Attic-Boeotian border until its inhabitants, disliking Theban pressure, traded their independence for Athenian citizenship. They brought a statue of Dionysus to Athens, where it was rejected; a plague followed, and the Athenians then instituted the Dionysia with a procession of phalloi. That festival gave rise to theatre, and the priest of Dionysus Eleuthereus held the centre seat at the Theatre of Dionysus.

The site sits on elevated terrain and requires decent fitness to walk around the walls and climb to the towers. Bring sturdy shoes and water, especially during warm months when sun exposure can be intense.

The fortress is also called Gyftokastro, "Gypsy Castle", a name it has carried since modern times. Its walls were built without mortar, and a traveller in 1879 noted that no grass grew in the joints. The sculptor Myron, known for the Discobolus, was born here, and his son Lycius was also a sculptor.