Gatehouse to Ardverickie Estate next to Loch Laggen ~ Scotland, Series "Monarch of the Glen" was filmed here. * would LOVE to incorporate a tower. Small, albeit, but a couple levels perhaps with telescope tower on top.
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
Windsor Castle in Berkshire, UK. Windsor is the oldest, longest inhabited castle in the world. Construction began in the century by William the Conqueror. It has been used as a residence by monarchs since Henry I.