The castle's bookings for large group tours already are filled for 2013. Individual tickets sell out practically as quickly as they become available.
The challenge for the current earl and his wife is harnessing "Downton's" popularity to support the massive house, which was built by the same architect who constructed Westminster Abbey.
Income from Lady Fiona's book, a New York Times best-seller for nonfiction, has helped. When "Downton" isn't filming (about six months a year), the castle staff also books weddings, corporate events and private dinners. It also has a new online gift shop for fans who can't visit Highclere.
Just like the "Downton" segment where Lord Grantham tells Matthew (the next Lord Grantham) about his responsibilities, including dozens of toilets, the Eighth Earl of Carnarvon and his wife worry about maintaining miles of pipes and acres of roof.
"Keeping Highclere up is a responsibility as well as a privilege," she says. "There's always something going wrong. We divide our tasks, with my husband taking responsibility for roof tiles, while I do the pretty curtains. But I've also crawled out on the roof with him to see where our money is going. It's not that scary, if you're wearing the right shoes."
Lady Fiona knew something about Almina and the Fifth Earl before she became Countess of Carnarvon. But she really delved into the couple when she injured her knee and spent hours on the library sofa. "I asked the staff to bring me things to read from the castle's archives. It was just after 'Downton's' first series had aired and its popularity gave me the chance to get Lady Almina's story published," she says. Now she's writing a second book about the castle's history, from 1923-1945.
The author and her husband live in Highclere during filming. But when summer crowds arrive, they move to a smaller home "about 20 yards away. We don't want to have to be up and out by 8 a.m. each morning," she explains.
No matter where they're living, they watch every episode, so they can answer questions when "the press rings up and wants to know our thoughts about a scene."
A dedicated dog lover, she admits to wondering why one of her Labrador retrievers wasn't tapped to be Lord Grantham's dog. (When the series began, she had two Labs and now has four, in addition to three cocker spaniels).
She has never wanted a "Downton" role for herself, however. "I see how hard the actors work and how long their days are. Most of the women spend their time in hair rollers and the men spend hours just waiting to go on. I have a very different view of what seems so glamorous from the outside. I'd rather take my dogs for a walk."
But it would be a different story if Lady Almina still was running Highclere. "Almina would have jumped at the chance for a part. She loved life and was so outgoing that I'm sure she'd have expected a part on par with the role played by Dame Maggie Smith," she says.
Want to go?
WHAT: England's Highclere Castle, where the PBS show "Downton Abbey" is filmed
WHERE: Highclere Park, Newbury, England RG20 9RN
WHEN: 10:30 a.m. to 1 p.m. (morning tickets) or 1 to 3:30 p.m. (afternoon tickets), Sundays through Thursdays, July 14 to Sept. 12. For additional opening dates (various holidays during the year), check the website.
HOW MUCH: Admission prices vary according to currency fluctuations; currently about $30 for adults, $26 for seniors and students, $18 for children; family admission (two adults and two or three children) $82. Admission includes tour of the castle, its Egyptian Exhibition, gardens and grounds.
GETTING THERE: Visit the castle by car or take a train from London's Paddington Station to Newbury's Rail Station. Taxis from Newbury should be booked in advance.
WHERE TO STAY: The castle's website lists bed and breakfasts nearby as well as some country inns. Want to hang out where "Downton" stars stay? Check out The Carnarvon Arms (www.thecarnarvonarmshotel.com), a short distance from the castle.
TIP 1: Do not think of going without procuring tickets in advance. Summer tickets go on sale in February and will sell out quickly.
TIP 2: Ticket times are for exploring the castle and Egyptian Exhibition. The grounds, gardens, tea room and gift shop are open throughout the day.
INFO: 011-44-1635-253204, www.highclerecastle.co.uk