High on a hilltop above Lostwithiel, 9 miles from St Austell on the A390, the ruined castle of Restormel was built by Edward the Black Prince (the first Duke of Cornwall), although he only stayed there twice during his life. It’s one of the best-preserved circular keeps in the country, and affords brilliant views across the river and fields from its crenellated battlements. Almost nothing remains of the interior, apart from some side rooms and a medieval garderobe (storage room).