Bourtzi is the small island that is situated in the east entrance of Poros.
The (Bourtzi) Island Castle was first recorded as being built in the times of Byzantium.
The Arab fleet took possession of a string of islands including Rhodes and Chios and was constantly harassing the Anatolian coast and penetrated into the Aegean. That is why and when the castle of Agios Constantine, with its church on the top, was built in 640 AD: to protect the valuable harbour and the hinterland and olive groves of Poros, Galatas and Trizinia.
Through the years the castle of Agios Constantine was owned by the Venetians and then the Ottoman Turks and was named Bourtzi. Finally in 1821 the Greeks took back the whole area in their Battle of Independence with the help of a coalition of Russian, French and British fleets.
Capodistria, the Governor of the Greece, set up his government at Trizinia and commanded the Bourtzi to be rebuilt in 1827 by a Bavarian, K Heideck, to defend the fleets. The church of Agios Constantine was still standing, under the supervision of the Monastery of Zoodochos Pigi on Poros. But the Architect Heideck was badly in need of stone so he dismantled the church on the promise that he would replace it after the monks had removed all the relics and Icons. But the church has never been rebuilt again.

This is an extract of an article by Peter J Pitt. Read the whole article here.