With season two of PBS’s blockbuster “Downton Abbey” reaching its climactic conclusion Sunday night, we may have just the cure for your period-piece hangover. You might not think it, but there are a number of majestic residences, much like Downton’s Highclere Castle, right here in the U.S. And many are actually for sale. TopTenRealEstateDeals.com found five castles, built over the last 200 years for various reasons, that are all for sale. Each has a history and beauty that are as intriguing as the estates themselves.