Restormel Castle or Kastel Rostorrmel lies by the River Fowey near Lostwithiel in Cornwall, England, UK. It is one of the four chief Norman castles of Cornwall. The castle is notable for its perfectly circular design. Although once a luxurious residence of the Earl of Cornwall, the castle was all but ruined by the 16th century. It was briefly reoccupied and fought over during the English Civil War but was subsequently abandoned.
Located on a spur of high ground overlooking the River Fowey, Restormel Castle is an unusually well-preserved example of a circular shell keep, a rare type of fortification built during a short period in the 12th and early 13th centuries. Only 71 examples are known in England and Wales, of which Restormel Castle is the most intact of all.