Prince William and Princess Kate have been happily married for over a decade, and according to a royal author, there's one key reason for that.

Having braved numerous challenges side by side, including the harrowing episode of Kate's cancer battle, there is one fundamental element that has kept their bond strong, as unveiled by a royal expert.

Christopher Andersen, author of The King, attributes their connection to their playful nature one trait they are also instilling in their children.

Speaking with Fox News Digital, he revealed: "William once described Kate's sense of humour as 'naughty' and his own as 'dirty', and claimed that it was one of the most important things that drew them together.

"They've certainly cleaned up their act for the children's sake, but they can still make each other double over laughing. William and Kate have also passed their wacky, slapstick, Monty Python-meets-Benny Hill sense of humour on to the children.

"And like all parents of young children, William and Kate find themselves refereeing when the kids fight over the TV remote, or when the inevitable teasing that goes on between siblings turns into the equally inevitable wrestling match."

They also have a shared sense of competitiveness, which the Duke reportedly found a little intimidating at first. James Middleton, during his chat on The Dog podcast, confided that Prince William sometimes found family game nights a little overwhelming.

Host Emily Dean said: "The Middletons are quite competitive and they love a game, don't they? And then you made poor William play, and he found it too competitive and actually used to make excuses."

In response, James shared: "It was after family dinner, we would be sat there and the cards would come out, and [he would] perhaps see if Ella [his dog] needed a walk or make an excuse to get down from the table. Racing demon is probably the most fast-paced, competitive one that we all play together."

James also penned in his book: "Ella gives [William] a good excuse to escape the fiercely competitive nature of the Middleton family, which emerges every time we play our favourite fast-paced card game, racing demon.

"William flinches at our ruthless determination to win at all costs. He's delighted to be the first out, and when no longer compelled to take part, he slinks off to cuddle Ella."

He also jokingly described the Prince's lack of competitiveness in cards, saying: "I think he'd prefer to absent himself from the game entirely. 'James, does Ella need a walk?' he asks before we've even started dealing the cards, and my sisters and I exchange a knowing glance: William, for all the competitive rigour of his military training, is happy to be a loser at cards."