Acquafredda Castle is an important testimony of fortified structure of Medieval times. It is 4 km from the town of Siliqua and stands on a volcanic hill, growing to a height of 256 meters above the sea level. With a decree of 1993, the site was named "Andesitic Domo of Acquafredda", Natural Monument. By the discovery of a Papal bull, dated July 30, 1238, in which Gregory IX gives measures to ensure that necessary measures are taken to put in battle fortifications are deemed of Torres, Gallura and Cagliari, it is believed, that Castle already existed since 1215, but it is widely attributed its construction to famous noble Pisan Ugolino Della Gherardesca. The story of Count Ugolino have become famous thanks to the verses of Dante Alighieri, in "Divine Comedy". After the people of Pisa, Castle became property of Aragonese. Like other castles built along the borders had a purely defensive function: the structure, very degraded today, it was part of a fortification built on three levels: village, water tower and castle itself. The village, at the foot of the complex, is defended by walls with battlements and included rooms used to serve, to the troops, stables and warehouses. The second level is still visible a cistern tower, with a barrel vault, and consists of three rooms that could be completely filled with water. Andesitic Domo of Acquafredda features lots of reasons of natural interest: historical, landscape, geology, fauna and flora make this place a natural site of great aesthetic value and of great scientific interest.