This is an ancient citadel that was active until the Venetian period. It is on a rocky hill above the sea, north of the village Vroutsis at the lower side of Amorgos (southwestern part of the island). The remains of a Middle Cycladic settlement which cover a long period from the Geometric up to the Roman era (inscriptions, sculptures, ceramics etc.) have been found on the rock of the citadel. The so-called "treasure of Arkesini" with Byzantine coins was discovered in the ancient city of Arkesini, and it serves as proof that the citadel continued to play a defensive role during the Middle Byzantine period.
The shape of the citadel is attributed to the natural rock carving (done in ancient times) to create a strong defensive position. On the rock you can see the preserved remains of Venetian buildings and fortifications. Apparently the citadel served as a fortress during the Venetian period (which ended in 1537 with the occupation by the Turkish pirate Hayredin Barbarossa). It seems, however, that during the latter period, it did not serve a strong Venetian castle but more like a shelter for the people who were looking for refugee from the pirate raids.
Also, on the rock there is the church of Panagia Kastriani (celebrated on September 8th).