I've not stayed in any of these but recently spent a night in one of the castles overlooking the Rhine, which was a fun experience. Back in the 80s—pre-internet and cell phones—we did a trip that basically involved staying at a variety of old castles, former monasteries, etc., in Germany. We'd lived there for several years earlier, so were comfortable with impromptu travel and didn't want to spend time touring museums but just relax and enjoy the countryside. We'd booked our first night at a castle hotel close to Fankfurt, where we found a brochure from the organization of castle hotels (which still exists). We also had a tour book that focused on unusual accommodations. So each day, we'd look at what neat hotel was a day's drive away from our current hotel, make a call from the landline to book a room, and then spend a relaxing day exploring our way there. At that time, none of them was super expensive and were in mostly rural locations. Still remember that trip fondly!