Penrith Castle

Admittedly not much of a castle, more a derelict bomb-site, but it's all we've got so we love it regardless.  Richard III lived here at times between 1471 and 1483, before he became king.  Considering it was already partly decayed in 1565 perhaps the most surprising thing about it is that it's here at all.  Especially when you consider the Cumbrian farmers' tendency to re-use bit of old buildings for new buildings, farm walls, etc.