Head to the small, sleepy town of Sedini to see one of the area's best-known domus de janas (prehistoric tombs). Gouged out of a huge calcareous rock known as La Rocca, the prehistoric tomb was lived in by farmers in the Middle Ages and used as a prison until the 19th century. It now houses a small museum displaying traditional farming tools and household items. Four weekday buses run to Sedini from Castelsardo (€1.90, 25 minutes).