In in the enormous "lake" of Roman Empire, the Mediterranean, Thira is nothing more than an insignificant island. The Christianity however reached early and it appears that, from the fourth century, existed already as an organised church. Is also testified as "Thira's Bishopric" with the first bishop named Dioskouros. From the period of Ioustinianos until 1207, when it was also suppressed, the Thira's Bishopric was one of the depended in the Metropolitan throne of Rhodes bishoprics and are always reported fifth in the line.

Neither political or military importance appears that it had in the Byzantine years. It is however remarkable that Alexios A Komninos (1081-1118) founded the temple of Virgin Mary of Bishopric in Gonia, perhaps as catholic some monastery.

From the years still Byzantines, the most terrible plague of Aegean islands were the every nation, race and language pirates, who undisturbed occupied the coastline imposing taxes and took whatever they wanted. Because of this and in order to rescue their lives, but also in order to safeguard minimal existing their from the invaders, people on the island had encompassed their villages in fortified "Castelia" (Castles), to which they resorted even the serfs when the sentinels gave the signal of alarm from the utmost sides of the island. In the island of Santorini, are reported - from the years of Frankish - five castles. The castle of Skaros was the most important, not only because of its location and equipment, but also because in this had been installed and resided the Fragoi souvereigns of island, with their Despot. Skaros, was built under the Imerovigli village, in the rough cliffs of "Theoskepasti". Today are rescued minimal ruins, because from 1700, residents abandoned Skaros and moved to Fira.

Second was the Casteli of Apanomerias (Above Side - Oia), that was named also "the Castle of Saint Nikolaos". Third was at Pirgos village, fourth, in Emporio village and fifth was the Casteli in Akrotiri, which it's named also as Pountas Casteli. The last four Castles were built in the homonym villages of the island and before the earthquake of 1956, they were all in good condition. Outside from the each Castle and exactly in the front side of his gate, it was (and it is still) a church which all of them are dedicated in the name of Saint Theodosia. Even in the castle of Skarou, which the most residents there were Catholics, existed at the front of the gate a church in memory of Saint Theodosia. It's worthwhile to mention Saint Theodosia is celebrated on 29 May and that the same season where her martyrdom was taken place in the Byzantium, the 726 m.H., became also the terrible explosion of volcano, that created Palaia Kameni(Old Burned).