Windsor Castle was raided by two masked men earlier this year, while Prince William, Kate Middleton, and their children slept in the estate's Adelaide Cottage.
It has been reported that two masked men scaled a 6ft fence to break into the royal grounds, home to the Prince and Princess of Wales and their three children, on October 13.
The thieves smashed through a security gate, which was the closest exit to their family home, while driving on a quad bike stolen from a barn.
They were reportedly aiming for Shaw Farm, with their eyes on the valuable vehicles stored there.
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The farm is an important part of the estate, as the Queen's funeral procession to Windsor Castle began from that very gate.
A source who spoke to the Sun said that the intruders 'must have been watching Windsor Castle for a while'.
King Charles and Queen Camilla were not at the residence at the time of the break-in, but the wrecked gate is an entrance that William and his family are seen using quite frequently.
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Gun cops are now patrolling the two main public entrances to Windsor Castle.
The source said: "They would have to have known the vehicles were stored there before they broke in and known when was the best time to get and escape without being caught.
"So they must have been watching Windsor Castle for a while."
They added: "There are alarms at Windsor Castle but the first time anyone knew there was a break-in was when they crashed through the security barrier at Shaw Farm Gate exit.
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"The barrier has needed to be replaced as it was so badly damaged."
It is presumed that Prince George, 11, Princess Charlotte, nine, and six-year-old Prince Louis were all have been sleeping in bed.
The source explained: "It was a Sunday night on a school week, so the young princes and princess would have been tucked in bed at Adelaide Cottage, just around the corner but still in the grounds."
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William and Kate moved to Adelaide Cottage in the summer of 2022.
They previously lived in an apartment in Kensington Palace, which they now use as an office when travelling to London for work.
On Sunday (November 17) night, a spokesman for Thames Valley Police confirmed: “At around 11.45pm on Sunday October 13, we received a report of burglary at a property on Crown Estate land near to the A308 in Windsor.
“Offenders entered a farm building and made off with a black Isuzu pick-up and a red quad bike.
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"They then made off towards the Old Windsor/Datchet area.
“No arrests have been made at this stage and an investigation is ongoing."
Topics: Crime, Kate Middleton, Prince William, Royal Family
Mia Williams