St Michael's Mount is a striking and imposing castle, perched high on an island just off the coast of Marazion in Cornwall. At low tide it's possible to walk across a stone causeway to the island. Lots more interesting info here
Castle Howard is a stately home in North Yorkshire, England, 15 miles north of York. One of the grandest private residences in Britain, most of it was built between 1699 and 1712 for the Earl of Carlisle, to a design by Sir John Vanbrugh.