I came across this site and am sure I could waste more time drooling over the profiles of these properties, but instead I'll just point you to the Patrice Besse company which lists quite a few castles and other historic buildings in France for sale.  Some of them are actually not as expensive as I thought they might be, including this 14th century church (450k euros) and this 15th century manor/castle (172k euros, but in ruins).  One of the coolest properties, though, is this 13th century castle in southern France – a “Cathar Fortress”.  What would I do with such a property, you may ask.  But if you have to ask, you're probably missing the whole point.  ;D