The ruins of the former Athonite Monastery of Zygos (Fragokastro) are located in the area of today's Ouranoupolis, about forty metres outside the current boundary line of Mount Athos. It is an ancient Athonite monastery, founded in the mid-10th century and destroyed shortly before 1198. Its entire fortified enclosure, reinforced with ten towers, which surrounds an area of 5.5 hectares, is preserved.

The first known reference to "Zygos" on the Athos peninsula comes from a document of 942. It is probably a basic topographical reference point in the area, but it is not specified whether it was a place, a village or a monastery.

In 991 it seems that the monastery of Zygos had already been founded, but the first testimony about it is only from 996 and it was dedicated to Prophet Elias.

During the 11th century the monastery was considered one of the most important monasteries, but in 1199 it was already deserted and was granted, as a metochi, by the emperor Alexios III Angelos to the then reconstituted monastery of Helandarion.

Around 1206 a Frankish lord and his soldiers settled in the castle of Zygos, who went on an exodus from there and plundered Mount Athos, until in 1211, with the intervention of the Pope of Rome, he was expelled from the area.

Excavations have shown that the monastery was built on a site where there were pre-existing facilities from the 4th century BC to the 6th century AD. The buildings consist of the old nucleus, which was doubled in size with an extension to the east. The Katholikon, located in the extension, began to be built at the beginning of the 11th century.

Initially the four-conical temple was built, with its narrow narthex. Later on, the northern chapel was added, with the tomb, then the six-narthex was added and finally the southern one-roomed trullae chapel with the tomb.

It was completed with the construction of three formal tombs in contact with the south wall of the Katholikon.

The walls of the temple are preserved at a height of 2 to 4 metres. The marble architectural parts of the temple, excellent works, which were not looted, were left dismembered. The four columns that held up the dome are missing, in contrast to the marble diaphragm of the north two-light opening of the nave, which is preserved in place, almost intact.

In the narthex, parts of the large representation of the Annunciation have been preserved, while in the south chapel there is a representation with a full-faced hierarch, probably of Saint Nicholas.

The floors of the Katholikon and the northern chapel are decorated with marble foundations, which are preserved in a satisfactory condition, probably works of the 11th century.

Among the excavated microfindings, the three lead dumplings of the 11th century are characteristic, a silver-gilt medallion with a depiction of St. Paraskevi, a small seal with a depiction of the Archangel, glass mosaic mosaic fragments, bronze needles and thimbles, knives, 11th and 12th century coins, glazed pottery and glass vases of the same period.