Not entirely sure what a Grade 1 castle is (hmm, the best?), but here's the listing for that Bath, England castle recently purchased by Nicolas Cage, a savvy investor who knows there's no castle bubble.
"An exceptional and unique small Country Estate with a magnificent Grade I Listed 18th Century castle. 2 Cottages, pasture and woodlands in all 58.60 acres. 3 Reception Rooms, 7 bedrooms, Extensive Lower ground Floor, Garages and Stores. 2 Cottages. Gothic Orangery and further traditional outbuildings. Chapel Ruins and former Priory. Parkland, Grassland and Woodland."
UPDATE: A Brit in our midst! Or at least someone who speaks the British real estate language explains "Grade 1" in comments.
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