Restormel Castle in Cornwall

You're standing at Restormel Castle, on a spur of land above the River Fowey, just outside the little town of Lostwithiel. First, take in the shape. This castle is almost perfectly round. It's what's called a shell keep: a ring of stone wall set on an earthen mound. Castles like this were only built for a short spell, in the twelfth and early thirteenth centuries. Seventy-one are known in England and Wales, and this is the most intact of the lot.