Restormel Castle is sited on a hill above the River Fowey, 1½ miles north of Lostwithiel. The Normans probably built the first castle in about 1100, but this wooden castle was replaced in the 13th century by a new stone construction.

Restormel has the best example of a shell keep in the country. The walls of the circular keep are over 2.4m (8ft) thick and 7.6m (25ft) tall, and are surrounded by a moat. Within this shell a series of buildings were constructed that butted up against the wall, forming the kitchen, great hall, and living quarters.

The stone keep would have been an impressive residence and was probably built as a symbol of status and power rather than a fortress. The only military action the castle ever saw was during the Civil War, many years after the castle had already fallen into ruin.