It's said that Queen Elizabeth is never happier than when she's safely ensconced at Balmoral Castle in the Scottish Highlands. 

The sprawling estate, which covers 20,000 hectares of moors, forests and farmland, has been in the Royal family since it was purchased by the Queen's great-great grandfather Prince Albert in 1852. 

The Windsors have long used Balmoral as their summer home, with the Queen staying there almost every year of her life from August until October. 

It was a place where the Queen could run free as a girl, catching salmon, stalking deer and attending local dances. 

Balmoral is also the place where her romance with a dashing British officer and distant cousin started to blossom. 

After their marriage, it was a private retreat where their children and grandchildren could join them for rowdy weekends. 

"I suppose Balmoral is a place one looks forward to very much," she said in 1992. 

"It is rather nice to hibernate when one leads such a movable life. To be able to sleep in the same bed for six weeks, it is a nice change."

Now her family has once again descended on their beloved retreat. 

But this time the mood may not be so jovial. 

With the Queen under medical supervision amid concerns for her health, her four children, including her heir Prince Charles, have rushed to be by her side. 

Love blooms on the moors

Even as a little girl, Elizabeth's annual trip to Balmoral was the highlight of her year. 

Her life changed at the age of 10 when her father suddenly became king, catapulting her to the top of the line of succession. 

As she prepared for her future, Balmoral was the one place she could be a normal child. 

"They looked forward to it all year round. It tended to be the chief landmark in their calendar," Marion Crawford wrote in her biography The Little Princesses. 

Her father brought his heir along with him on deer-stalking expeditions. 

"It was always fun to see a new stalker out for the first time with the Queen," her cousin Margaret Rhodes told biographer Sally Bedell Smith. 

"She would be crawling on her stomach with her nose up to the soles of the stalker's boots, which would be a surprise to the stalker."

She met her future husband Prince Philip at a family wedding in 1934. 

But as a regular guest at Balmoral, the couple's love bloomed in the rugged Scottish highlands. 

"The young people went out with the guns and picnicked together, but they were very seldom alone," Marion Crawford said. 

"Occasionally he would take her out for a drive, and now and again they would manage to get off into the gardens after tea." 

Prince Philip told his biographer Ingrid Steward that it was at Balmoral in 1946 where he and the Queen started discussing marriage. 

"It was probably then ... that we began to think about it seriously, and even talk about it."

The couple became engaged at Balmoral by the summer's end, though they delayed a public announcement until she turned 21 the next year. 

A family home that can be 'intriguing, surreal, utterly freaky' 

The grandeur of Balmoral is said to be slightly overwhelming for first-time visitors. 

With turrets, 52 bedrooms, draughty corridors, tartan rugs and walls mounted with antlers, it can be an intimidating place. 

Every morning, a lone piper plays below the Queen's bedroom window.

"There is a certain fascination in keeping the place as Queen Victoria had it. Nothing very much has changed," the Queen said. 

For decades, it has been practically mandatory for the British prime minister to pop up to Balmoral for the weekend once a year. 

But not all of them have enjoyed it. 

Margaret Thatcher reportedly referred to the estate as "purgatory".

And John Major found the bagpipes played every morning to be endlessly frustrating as he tried to make phone calls to other world leaders. 

Tony Blair has described his annual visits to Balmoral as "a vivid combination of the intriguing, the surreal and the utterly freaky". 

But even with their regal surroundings, Mr Blair was slowly won over by the Royal family's casual traditions at the estate. 

He said the Queen insisted her PMs join the family for parlour games, plied them with strong drinks that tasted like "rocket fuel", and ordered Prince Philip to man the barbecue. 

"You think I'm joking, but I'm not," he revealed in his memoir. 

Balmoral may be the one place that the Queen can be truly herself in a tightly controlled existence.

"I think Granny is most happy there. I think she really, really loves the Highlands," said her granddaughter Princess Eugenie in 2012. 

"Family-wise we're all there, so it's a lovely base for .... walks, picnics, dogs — a lot of dogs, there's always dogs — and people coming in and out all the time." 

Amid the grand furnishings of the castle is said to be an electric heater to keep the monarch cosy, and a pillow cheekily embroidered with the words: "It's good to be Queen". 

'At Balmoral, she hadn't taken it in' 

While it might be the site of some of the family's happiest memories, Balmoral also looms large in one of their greatest tragedies. 

On August 31, 1997, the Queen was woken up in the early hours to be told that Prince Charles's ex-wife Diana had been in a car accident. 

With William and Harry asleep down the hall, the Queen, Philip and Charles paced the halls waiting for news.

At 4am, a palace aide came in to tell Charles that the mother of his children had died. 

The Queen acted quickly, ordering the televisions and radios to be removed from the nursery and insisting the boys be allowed to sleep until daybreak before being told.