Security plans at Windsor Castle marked confidential have revealed the Sovereign’s private staircase leading onto the south terrace (Picture: Getty/The National Archives)

Security plans for Windsor Castle have revealed a maze of hidden areas and passageways – including a private staircase linking the King’s quarters to the outside world.

The declassified maps showing underground chambers and tunnels were used for a wide-ranging security upgrade in the 1960s and have been newly released at The National Archives.

Marked ‘confidential’, the drawings show a discreet staircase tucked away to the rear of the Sovereign’s entrance in the south-east corner of King Charles III’s residence in Berkshire.

Interconnected subways, bricked-up areas and chambers are also shown in the bird’s eye plans of the castle, which dates back to the 11th Century.

A large wine cellar is also shown at basement level.

The maps were used for the security review which included an upgrade to the alarm system, as covered by Metro.co.uk last month.

The plans also show various features designed to keep out intruders, such as a control room and an alarm bell.

Plans marked confidential show the layout of Windsor Castle with the coloured dots representing security features (Picture: The National Archives)

The map of Windsor Castle used for a security review to guard against intruders and thefts (Picture: The National Archives)

In 2011, the first ever publicly broadcast footage inside a secret escape tunnel at the castle followed Fiona Bruce walking through the passageway.

The BBC presenter showed viewers a trap door concealed under a carpet in an office.

She said: ‘This is an office just tucked away in a corner of Windsor Castle.

‘But look under here. As if by magic, just lift these and the medieval castle emerges.’

The Sovereign’s private stairs can be seen in the south-east corner of the map with a red dot marking a detection point (Picture: The National Archives)

Walking down the stone steps, Bruce provided insights into the rich history of the working palace and private home, which spans 484,000 square feet of space.

Founded by William the Conqueror and originally used as a fortress, the castle has many layers of intrigue and history, some of which are open to visitors.

Bruce said: ‘If you’re a soldier in Windsor Castle under siege you need a way to get out. And this is the secret passage.

‘This is exactly what it looked like in the 1200s.

‘It’s wide enough to accommodate a whole army of men. You can just imagine them rushing down the stairs, and it leads out onto the street.

‘This is the clever bit – they’d then be able to sneak up on the enemy and attack them from behind.’

Fiona Bruce in the tunnel at Windsor Castle intended as a secret escape route from the landmark (Picture: BBC One)

It’s not clear if the tunnel is the same as the private staircase shown in the newly-released plans, which were originally drawn up in 1935 by HM Office of Works before being used to map out the security upgrade.

The drawings show the ‘sovereign’s private stairs’ which lead out onto the south terrace from the Victoria Tower. The subway systems have also not been marked with a precise description, although in places they appear to have been used for carrying cold air around the buildings.

Leslie Grout, a historian and Mastermind champion, has extensive knowledge of Windsor. He cited architectural historian William St John Hope’s three-volume history of the castle which was published in 1913.

‘There are places in the castle that are not open to the public for reasons such as security and physical accessibility,’ Mr Grout said.

‘William St John Hope has shown a number of them, and a few more may have been discovered since then during building and restoration work.

‘One example is a medieval rubbish pit that was discovered after the fire.

‘As to what they might find in the future, one never knows.’

Mr Grout told Metro.co.uk it might not only have been the sovereign who required a discreet way to slip in and out of the castle.

Subways and chambers can be seen along with a large wine cellar in the basement of Windsor Castle (Picture: The National Archives)

A wine cellar is accessible by long sloping passageways on either side of the chamber in plans of the castle’s basement (Picture: The National Archives)

‘Charles II may have wanted his mistress Nell Gwyn and others to enter and leave the castle unobserved,’ he said.

‘But I’m not confining it to Charles II, there are other monarchs who may have wanted to keep their guests from public view.’

What is in no doubt is that the 1,000-year-old landmark is a place of many secrets. A light into some of the untold history was shone last month when a dossier was released showing details of intruder incidents in the late 1960s.

The list included a party of RAF personnel who crept in with the intention to place a flag on the Round Tower or remove a cannon from the East Terrace.

Another incident involved an intruder using a trade door at a porter’s lodge to slip unnoticed into the royals’ private apartments.

A general view of people on The Long Walk outside Windsor Castle (Picture: Chris Jackson/Getty Images)

The reports, together with the plans, were contained in the dossier marked ‘secret’ which was released at The National Archives last month.

The upgrade was carried out after concern among palace officials that treasures within the castle worth millions of pounds were vulnerable due to the ageing security features.

Windsor Castle is not the only royal residence said to have an intriguing history below ground level.

Buckingham Palace has long been rumoured to have several secret tunnels and doorways — including a passageway that leads down The Mall to Clarence House.

MORE : Declassified files reveal ‘large number’ of security scares at Windsor Castle

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A warren of passageways and chambers are hidden at the royal residence which date back 1,000 years.