Castle-an-Dinas is one of the largest hillforts in Cornwall. It is located on the summit of Castle Downs with views across central Cornwall to the north and south coasts. In Cornish legend it is said to be one of the seats of the Duke of Cornwall where Cador, Duke of Cornwall, and Ygraine, King Arthur's mother were killed. The hillfort is surrounded by three ramparts and ditches. There are two Bronze Age barrows in the interior, which could have been the initial use of the site and around which the enclosures were built. However, it is thought that the site could be Neolithic in origin.