The construction works for the Venetian Castle of Folegandros also known as Kastro began around 1215, a few years after the end of the Fourth Crusade. The medieval defensive settlement was built on the edge of a steep cliff, around 200 meters above the sea and functioned similarly to a defensive fortress, designed to protect the small town from pirate attacks. Local juniper trees were used to support the roofs of the small houses and the exterior walls of these houses had not been painted so that it would be more difficult to spot them from the sea. Some of the houses have been reconstructed but during the war, they used to have escape passages and embrasures. The remnants of older walls were used as a foundation in order to build the houses that are still inhabited these days.
This historic part of the town is exceptionally beautiful, with a unique architecture, whitewashed walls and blue shutters. The whitewashed stairs that were once used as seating areas by the former inhabitants of the tiny houses are currently decorated with pots and colorful flowers.