The castle-monastery of Saint John is a unique monument of Orthodoxy. It overhangs the whole island as it is built on the top of the hill right across the island’s port, Skala. Around it is the medieval settlement of Chora.
The monastery dedicated to Saint John Theologos was built in 1088 by Osios Christodoulos, to whom was given the island by the Emperor of Byzantium Alexios Komninos A’.
The imposing monastery has the architectonic construction of a byzantine castle. That way it could be protected by pirate invasions.
Inside the monastery’s walls you will see the byzantine church of Saint John, which is decorated with important murals from different periods of time. Highly impressive is the wooden chancel screen in the church. Among the Byzantine icons distinguishes that of Saint John Theologos, donated by emperor Alexios A’ as and the icon of Saint Nikolaos with tesseras. The church’s Chancel table is made by a stone from the ancient temple of god Artemis.
The church has two chapels. One is dedicated to Osios Christodoulos whose remains are saved in the church’s reliquary and the second is dedicated to Virgin Mary.

THE MONASTERIES MUSEUM
The ecclesiastic museum roofed under the church of Saint John, hosts priceless religious monuments. Among others you will see sacred episcopic canonicals brocaded with precious stones. Sacred appliances, precious crosses, sacred communion cups, emperors’ crowns and forked caps.
Among the precious exhibits are the crown of emperor Alexios A’ and the mitre of the ecumenical patriarch Neophytes the 6^th made by 3k. of gold bejeweled with precious stones.

THE MONASTERIES LIBRARY
Very important is the monastery’s library built by Osios Christodoulos himself, to whom the first books belonged to.
Today this library is one of the biggest in Greece. It has over 2000 tomes of old editions and 13.000 copyrights of various documents. It also has 900 manuscripts from which 325 are written in scrolls.
From these most important are: the Porfir Codex, semi-finished copyright of a gospel that belongs to Evangelist Marcus, Iov’s book, manuscripts of the 7^th and 8^th BC, the donation act of Alexios Komninos A’, through which the monarch donated Patmos to Osios Christodoulos and many documents of byzantine emperors, patriarchs, princesses and other officials.