Birkhall is on the Balmoral estate in Aberdeenshire and originally belonged to the Queen Mother, but was inherited by Charles. The Prince of Wales and Camilla, Duchess of Cornwall, like to retreat there for country pursuits like fishing and shooting.
However, on occasion he lends it to other members of the family and in the past he even lent the use of his ghillies – someone who acts as an attendant on a hunt or fishing expedition – so the visiting Middletons could enjoy a shooting weekend with Kate and William.
For example, in 2010 shortly before the engagement was announced, the Middletons were seen receiving instructions in shooting ahead of a day’s stalking in the Balmoral hills.
This indicated that the normal, middle-class family were being taught to fit in with the leisure activities enjoyed by the royals.
It was all part of a “country pursuits” house party that William hosted annually.
A source told the Daily Mail at the time: “William and Kate do go up to Balmoral quite a bit, but inviting her parents is a big deal. This is virtually confirmation they’re his in-laws-to-be.
“Carole must be absolutely ecstatic. She’s finally made it right inside the royal circle.”
This was not the only visit either, in fact Carole has visited the Scottish estate as recently as 2014.
However, according to a 2016 article in the Daily Mail, growing tension brewed between Charles and the Middletons over time spent with their mutual grandchildren.
According to royal expert Katie Nicholl, the prince believed Carole and Michael get “more than their fair share” of time with Prince George.
In retaliation, it seems, the Middletons received fewer invitations to royal events and to Birkhall.
One of Charles’ circle said: “At the moment, there’s no talk of them going [to Birkhall].
“[Charles] wants to see his son and daughter-in-law and spend time with his grandchildren alone.
“He has always wanted to mentor George, who is, after all, the heir to the throne, and sees it very much as his role.
“He will always insist the grandchildren come to Scotland so that they can learn to shoot, hunt and fish.
“That’s very important to Charles. He wishes the family would do more traditional things like holiday in Scotland.”
All this was reportedly the result of a feud branded the “Battle Royal”, in which Charles apparently raged: “They never let me see my grandson”.
He even tried to entice William and Kate to spend more time with him by refurbishing a treehouse in the Highgrove garden for the children, and installing a £20,000 shepherd’s hut in the wildflower meadow.
However, both the treehouse and hut have been sadly empty, as the visits to Highgrove are few and far between.
This is partly due to how busy both Charles and the Cambrdiges are – conflicting schedules means it is hard for them to fit in quality time together.
The Middletons on the other hand have a lot of free time and can work around when the family is free, and even take on some of the childcare responsibilities when William and Kate are busy on engagements overseas, for example.
Carole was said to be “devastated” to find out that Charles felt this way.
A friend told Ms Nicholl: “All she had ever wanted is to be a good mother and grandmother, not upset anyone in the process.
“Perhaps their low profile is their way of keeping a respectful distance.”
According to the Daily Mail, a spokesman for the Palace declined to comment on the alleged souring relations.