Agios Nikolaos (Saint Nicholas) is one of the five medieval castles built to protect the settlements of Santorini from pirates. The unimpeded view allowed the residents of Oia to scan the horizon for enemy ships. The 1956 earthquake destroyed most of the citadel, except for the small church of Agios Nikolaos. The site also includes the remains of the old Church of Panagia Platsani and the goulas (residential keep) at the citadel’s highest point.