The British Army is investigating the incident as a matter "of priority" before Queen Elizabeth's Platinum Jubilee.
Traditionally, the Royal Guard is responsible for protecting the sovereign's palaces. However, according to Britain's latest report, the guardsmen were reported to have been friendly to one intruder and allowed him to spend the night on the grounds of Windsor Castle.
According to The Telegraph the incident occurred last Wednesday night. A man pretending to be a priest received royal treatment at Windsor's Victoria Barracks. These barracks house Coldstream Guards, a battalion best known for their red coats and bearskin caps.
According to The Telegraph, the intruder was reported to have gained trust from senior guards and been allowed to eat at the barracks.
The alarm was raised with the police and the intruder was taken from the scene. According to police, no arrests were made. The barracks were not located on the castle grounds.
A spokesperson for the British Army told NBC News that "The Army takes this security breach extremely seriously" and would be investigating it as a matter priority. "This incident is currently part of an ongoing investigation, and it would be inappropriate to comment further at the moment."
According to The Telegraph, a local pub landlord claimed that the same man entered his bar "claiming to be Prince Harry's friend in America" and asked for free food.
The security breach occurred at Windsor Castle, and Queen Elizabeth II wasn't there.
Windsor Castle has been the setting for many important royal events over the past few years. This includes the marriage of Prince Philip and the Duchess of Sussex in the chapel at the residence in 2018 as well as the funeral of Prince Philip last year.
To the south is the Victoria Barracks.
Although there has been no official statement from Buckingham Palace about the security breach, this comes at a moment when all eyes are on Queen Elizabeth II. This year marks her 70th year on the throne. It is the longest reign of any British monarch. A host of celebrations have been planned to honor her Platinum Jubilee.