Cyclades were under threat from pirates who plundered the harvests, enslaved men and women and sold them in the slave markets. The small bays of the island were also ideal as hideouts. In response the islanders built their settlements at the highest most inaccessible points, and very close to, or on top of, each other; while their external walls, devoid of openings, formed a protective perimeter around the village. In addition the following additional types of fortifications were built throughout the island to protect the island's inhabitants. (castles), also written as kasteli, were large fortified permanent settlements. There were five on the island, Agios Nikolaos (at Oia), Akrotiri, Emborio, Pyrgos, and Skaros. At the entrance to every was a church dedicated to Agia (St.) Theodosia, the Protector-Saint of castles. (from the Turkish word which means 'tower') were multi-storey, rectangular, and the highest tower of most kastelli. There were four goulas on the island. They were used both as an observatory and as a place of refuge for the islanders. They had thick walls, parapets, an iron gate, murder holes, and embrasures. were small coastal watchtowers, which were permanently garrisoned, from where a watch was maintained and an alarm raised when a pirate ship was sighted. Infrastructure Electricity Electricity for both Santorini and Therasia is principally supplied from the Thira Autonomous Power Station which is located at Monolithos in the eastern part of Santorini. Owned by Public Power Corporation (PPC) it has generators powered by diesel engines and gas turbines. The two islands have a total installed capacity of 75.09 MW of thermal generation and 0.25 MW of renewable generation. There is a programme underway at a cost of €124 million as part of the Cyclades Interconnection Project to connect the island via a submarine cable to Naxos and hence by extension to the mainland system by 2023. A fire at the power station in Monolithos on 13 August 2018 put it out of service, resulting in a total loss of electricity supply across the two islands. Within four days electricity had been restored to all but 10% of the islands' consumers. Vessels were dispatched to carry two power generators to assist in supporting the restoration of the electricity supply. Electricity is distributed around the island by The Hellenic Electricity Distribution System Operator (HEDNO S.A. or DEDDIE S.A.) which is a 100% owned subsidiary of PPC. A cable connects the Thirasia