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
Merlin's Well , St Nectans Kieve - This magical waterfall at the top of Rock Valley is one of the few places where it is possible believe completely in the legend of King Arthur. Legend has it that the Knights met here to discuss the Quest for the Grail.