Queen Elizabeth II was once a guest at the elegant Highclere Castle that served as the backdrop of the hit television series Downton Abbey.

鈥淢y father-in-law was one of her greatest friends from the age of 16, and my husband is her godson,鈥 Lady Carnarvon told CTV News from the manicured grounds of . 鈥淚t was a retreat and a haven where nobody ever talked. It was a place where she could enjoy the countryside.鈥

A begins Thursday to mark Queen Elizabeth II鈥檚 70 years on the throne, the longest reign of any British monarch. Events are scheduled across the United Kingdom, and beyond.

鈥淪he has been an extraordinary example, I imagine, of both firmness and directness, but above all of compassion,鈥 said Lady Carnarvon, also known as Fiona, the eighth Countess of Carnarvon.

鈥淚 think World War Two, which was the time she grew up in, we were all in the trenches,鈥 the countess added. 鈥淚t didn鈥檛 matter whether you鈥檙e the King or the Queen, or you were a farm labourer or a teacher, it was a huge leveller, which is completely correct, as was the pandemic.鈥

Lady Carnarvon鈥檚 husband is the eighth Earl of Carnarvon, and Highclere Castle is his family seat. They oversee the grand estate, located 100 kiilometres west of London, which was the primary filming location for all six seasons of Downton Abbey, plus two movies based on the series. The latest film was released in Canada on May 20.

First built in 1679, the 300-room castle was renovated extensively in the 1840s. Lady Carnarvon鈥檚 family mostly lives in a smaller cottage on the sprawling grounds, which are open for visits, guided tours and even . Past guests include Downton Abbey creator and writer Julian Fellowes.

鈥淗e鈥檚 stayed here, he鈥檚 eaten here, he鈥檚 drunk here, he鈥檚 slept here, so it gives it that sense of reality,鈥 Lady Carnarvon said.

Not all past visitors have had connections to the show or royalty. The castle was a hospital during the First World War and a home for evacuee children during the Second World War. Lady Carnarvon has telling its story.

鈥淗istorically, it鈥檚 not particularly accurate,鈥 she said of Downton Abbey. 鈥淏ut it wasn鈥檛 trying to be a historical documentary: Downton Abbey was supposed to be a costume drama and fun and interesting.鈥

The castle even has a Canadian connection. In , Lady Carnarvon describes how the fourth Earl of Carnarvon hosted Canada鈥檚 first prime minister, Sir John A. Macdonald, and others at the property to work on the , which led to the creation of the Dominion of Canada on July 1, 1867.

鈥淚t is the wealth of friendship, and the common grounds that we all have, and a reason to come together every so often,鈥 Lady Carnarvon said of the 54-member Commonwealth, which are mostly former British colonies like Canada. 鈥淭hat鈥檚 sort of invaluable, actually. So, I truly hope that people do stay with it.鈥

Lady Carnarvon has introduced a 鈥楩riends of Highclere Castle鈥 program, which is a two-tiered membership for people who are passionate about the building. , membership includes private tours of new rooms, virtual 3D tours, as well as access to parts of the building used for filming Downton Abbey.