So on 9 April 2005 - exactly 19 years ago today - they got married in a small, private civil ceremony in Windsor.

While their marriage is entering its 20th year, their relationship stretches back five decades to the early 1970s.

Now, more than a year into their reign, life for the King and Queen is very different. Here we look back at their relationship and how it might weather the challenges they face from now on.

Failed relationship first time around

Prince Charles and Camilla Shand are thought to have met for the first time at a polo match in London in 1970.

Camilla, the daughter of an esteemed military officer, had been in an on-off relationship with Andrew Parker Bowles, a captain with The Blues and Royals regiment of the British Army.

Charles had only been officially invested with the title of the Prince of Wales a year earlier, and was fresh out of Cambridge University and RAF training.

Having bonded over a shared love of polo and countryside pursuits, they dated for around two years, before the prince left to join the Navy and Camilla rekindled her romance with Mr Parker Bowles, marrying him a year later in 1973.

Over the years, many have cited the now King's military commitments as the reason their initial relationship broke down.

But Michael Cole, former BBC royal correspondent and ex-spokesman for the late Mohamed Al Fayed, recalls it differently.

"It would be wrong to say that he 'missed the bus' and could have married her then, but hesitated," he tells Sky News. "The fact was she loved Andrew Parker Bowles."

He adds that at that point, Camilla would not have been considered by the Queen and her advisers to be a suitable bride for the heir to the throne because she had a "past" (as it was put then) - meaning earlier relationships before meeting Charles.

'Third person' in Charles and Diana marriage

In the years that followed, the young Prince Charles was under pressure to marry, and began dating Lady Diana Spencer, the younger sister of his-ex girlfriend Sarah.

By that stage, Camilla had given birth to two children, Tom in 1974 and Laura in 1978.

Diana famously told Newsnight that despite still being married to her first husband, Camilla was the "third person" in the marriage.

This was evidenced by the "Tampongate" tape. Recorded by a radio enthusiast by accident six years before the Parker Bowles's divorce, it captured Camilla and Charles exchanging sexual innuendos.

It wasn't published until 1993, the year after the Prince and Princess of Wales announced their separation.

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Charles Anson, former press secretary to Queen Elizabeth II from 1990 to 1997, says that while it wasn't palace business to be commenting on private relationships, it was an "issue" that had to be navigated carefully.

"It was a feature of life at that time and therefore something that needed to be handled," he tells Sky News. "Prince Charles and Camilla were part of the landscape."

According to Mr Cole, it was always Charles driving their relationship in the early days.

"She was happy with her life in the countryside, with her children, and would have been quite happy to remain his mistress - she didn't expect anything else," he says. "But for Charles it was non-negotiable, he had to have her."

Going public

The breakdown of Charles and Diana's marriage dominated headlines as one of the biggest news stories of its time.

Revelations came in the form of Andrew Morton's Diana biography, the princess's BBC Newsnight interview, the publishing of the "Tampon tapes", and Charles's confession he had been unfaithful after the marriage "irretrievably broke down".

A month before Diana's death in Paris in the summer of 1997, Charles threw a birthday party for Camilla at his Gloucestershire country home, Highgrove.

The late Queen did not attend, reportedly only agreeing to formally meet Camilla in 2000.

It wasn't until after Diana died that Charles and Camilla officially stepped out in public together - at a birthday party for Camilla's sister Annabel Elliot at the Ritz Hotel in early 1999.

"It was all done gradually," Mr Cole says. "Step by step, the idea that they were together was introduced to the public."

Camilla had to navigate being gradually brought into the Royal Family as she faced media scrutiny and a tide of public opinion against her.

"It's a mixture of constitutional arrangements and very human relationships," Mr Anson says. "I think it was difficult to manage that, but it had to be done."

In a rare interview with British Vogue in 2022, Queen Camilla recalled: "I was scrutinised for such a long time that you just have to find a way to live with it. Nobody likes to be looked at all the time and criticised. But I think in the end, I sort of rise above it and get on with it."

The wedding at the Windsor Guildhall was a muted affair. Charles didn't wear his military garb and Camilla didn't wear white.

The late Queen didn't attend the ceremony but was there for the reception at Windsor Castle.

Mr Cole says: "Buckingham Palace had a real fear they would have eggs thrown at them, so the ceremonial parades were kept to a minimum."

Camilla, out of respect for Diana, took Duchess of Cornwall as her title, not Princess of Wales. It wasn't until 2022 that the late Queen ruled that Camilla would replace her as Queen in the event of her death.

'Hold each other up'

With time, both King Charles and Queen Camilla have enjoyed a considerable increase in public popularity.

Queen Elizabeth II famously referred to the Duke of Edinburgh as her "strength and stay" during their 73-year marriage.

Mr Cole describes Charles and Camilla as being "bookends" to one another. "They hold each other up. They're very devoted to one another," he says.

Mr Anson describes their relationship as the "bedrock of the monarchy" in recent decades.

"It's quite a tough environment for anyone to operate in," he says. "But the King and Queen share a good sense of humour, they share the successes and the setbacks, and that partnership makes the job a lot easier to do."

Both royal experts stress the Queen's "stoic" character given all she's had to endure, including the King's recent cancer diagnosis, and cite her close relationship with her children and grandchildren as a source of strength.

Her close sense of family will likely have provided comfort to the Prince and Princess of Wales as they navigate her cancer treatment, Mr Anson adds.

On Camilla, Mr Cole says: "She's always done what she's had to do, because that's what women of her background have always done. She's prepared to put up with it all."

He believes the King's marriage is just as strong as his mother's was.

"The Duke of Edinburgh always suffered from being two steps behind the Queen," he adds.

"But Charles is a lot happier in himself. He's got the job he's waited 70 years for and he's had a very successful first year on the throne."

Mr Cole says that while Charles and Camilla may have only been married for 19 years, "they've been together for 50".

"It's a long and impressive love story without any doubt - not a conventional one - but ultimately they've triumphed," he adds.