Lydford Castle, Devon

English Heritage
Lydford Castle is a small keep of Norman origin, rising above a mound in the middle of the village. In fact, the castle is not built on the mound – the mound was constructed around the lowest level of the structure when it was modified in the 13th century. It was used as a notorious prison for several centuries. It is now ruined and roofless.
The castle is worth a look if you are in the Lydford area. Apparently there are other Norman and Saxon defensive works to be seen nearby, so the downloadable audio tour to Lydford Castle and Saxon Town seems worth having.
Open at any reasonable time.