- The ruins of the monastery of Zygou (Frangokastro) are located in the area of today's Ouranoupoli, almost 40 meters outside of today's borderline of Agio Oros. Is an ancient monastery, which was founded in the middle of 10th century and destroyded a bit earlier from 1198. All his fortification precinct is preseved, reinforced with 10 towers, which covers an area of 5.5 acres.
- The first known reference "of Zygou" comes from a document of 942. It is rather for the basic topographical reference point in the area, but without specifying whether it was a place, a village or a monastery.
- In 991 it appears that the monastery of Zygos had already been established, but the first testimony was just 996 and was dedicated to Prophet Elijah.
- During the 11th century, the Monastery was considered one of the most important monasteries, but in 1199 it was already deserted and granted, as a shareholder, by Emperor Alexios III Angelos to the rebuilt, then, monastery of Chelandarion. During 1206, a Franciscan ruler with his soldiers settled in Zygos Castle, who routed out and plundered Mount Athos until, in 1211, with the intervention of the Pope of Rome, he was expelled from the region.
- Excavations have shown that the monastery was built in a place where pre-existing facilities existed in the 4th century BC. until the 6th century AD. century. The buildings consist of the old core, which doubled with eastward extension. The Katholikon began to be built in the early 11th century.
- Originally, the four-aisled temple was built, with its narrow splint. Later, the northern chapel was added with the owner's tomb, then the exonarthex was added, and finally the south one-room trout chapel with the owner's tomb.
- Its completion was accomplished by the construction of three official tombs in contact with the south wall of the Katholikon.
- The walls of the temple are saved at a height of 2 to 4 meters.
- Its marble architectural parts, excellent works, which did not loot, were dismembered.The four columns that held the dome, are absent in contrast to the marble diaphragm of the north opening of the main temple, which is preserved in it's place, almost intact.
- In the narthex, parts of the great Evangelism have been preserved, while in the southern chapel a representation with a full-bodied hierarch, probably Saint Nicholas.
- The floors of the Katholikon and the northern chapel are adorned with marble statues, which are preserved in a satisfactory condition, probably works of the 11th century.
- From the excavated foundings, characteristics are the three lead seals of the 11th century, book closures,a silver gold-plated medal with a carved representation of Saint Paraskevi, a small seal with the representation of an Archangel, glass inlays from a wall mosaic, copper needles and thimbles, knifes, coins of the 11th & 12th century,ceramic and glass vases of the same age.
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