The ruins of the former monastery of Zygos (Fragokastro) are located near the current area of Ouranoupoli, about forty meters away from the current border of Mount Athos (Agion Oros). It is an ancient monastery, which was founded in the mid-10th century and was destroyed shortly before 1198. The whole fortification wall is saved up today, which is reinforced with ten towers and extends in an area of 5.5 acres.

The first known reference of "Zygos" at the Athos's peninsula comes from a document of 942. It is rather a basic benchmark of the region, without specifying whether it was a place, a village or a monastery. 
In the 991 seems to have already established the monastery of "Zygos", but the first known testimony it is just from 996 and it was dedicated to Prophet Elias.

During the 11th century, the monastery was regarded as one of the most important monasteries, but in 1199 it was already deserted and given up, as a monastic land, to the emperor Alexios III Angelos in the refounded monastery of Chelandariou.

Around 1206 a Frank lord with his soldiers moved to the Castle of "Zygos", who attacked and ravaged the Mount Athos (Agion Oros) from there, until 1211, who, with the intervention of the Pope of Rome, was expelled from the region.