Since the Byzantine era, one of the most crucial problems most Aegean islands had to face was piracy. Therefore, inhabitants used to build castles in order to find shelter and protect themselves at the sound of an alert signal. As it has been reported, since the years of Francocracy in Thera there have been five castles which are estimated to have been built between the 14th and 18th centuries. Skaros in Imerovigli was the most powerful one. The castle (Kasteli) of Epano Meria – renamed as Oia some years ago - was rather significant as well and it was named after the church of St. Nicholas. The three other castles worth mentioning were the one in Pyrgos, the one in Emporio and, finally, the one built in the settlement of Akrotiri. During the last four years before the disastrous earthquake of 1956, these castles were rather well maintained.
What mainly characterised these castles were the towers/observatories that were designed for defence purposes and were called 'Goulades' (a term that derives from the Turkish word 'koules'). They had multiple floors, rather dense walls and rectangular floor plans. They were considered as people's last resort in cases of attack. Goulades were situated either in the castles (as in Oia and Akrotiri) or outside of the castles (as in Fira and Emporio) so that the farmers could take refuge in there.