Sibenik Croatia
[Orthodox]
One of four Sibenik fortresses, St. Nicholas is the only one at sea. Built in the 16th century, it displaced the Benedictine Monastery of St. Nicholas that gave the fortress its name. It is on a small island at the entrance to the St. Anthony Channel. The army abandoned the fortress in 1979. It is now a UNESCO world heritage site and is undergoing significant restoration. Plans include a pedestrian walkway for access from the mainland as it's only been accessible by sea.