The Library at Fonthill Castle. Fonthill Castle (now a museum) is situated on 60 acres of land in Doylestown, PA. It was built by Henry Chapman Mercer, completed in 1912, and used as his residence. The "castle" consists of 44 rooms : 10 bathrooms, 5 bedrooms, 18 fireplaces, and an Otis elevator. Since 1976 the Bucks County Historical Society has administered Fonthill Castle Museum.