From the information plaque at the entrance: This building, in all probability dating back to the 14th century, has certain features which are difficult to interpret stylistically. Its name derives from the spiral staircase contained inside the unusual cylindrical tower rising on the eastern side. The castle is composed of stone stronghold towers, the tallest of which has a jutting-out crenellated upper section supported by stone wedges. Similar architectural features are also evident in the cylindrical tower which is topped by a sentry box, giving this building a rather unique and unusual appearance when compared to other fortifications of its time. The castle received a mention in 1555 due to the resistance it put up to the imperial French troops who finally brought about the surrender of Siena. http://flic.kr/p/q587uS