If you walk down Albany Street from the bridge leading to Regent's Park, you will find on your left a high, forbidding brick wall. It continues, uninterrupted for a long way. Finally, an arch appears, with a boom across the entrance. Above the arch is the sign: "Regents Park Barracks."

No contemporary photographs can be found of the interior of the barracks, but some can be found in archives and old newspapers.

The complex of buildings was constructed in 1820-21 as cavalry barracks for the Lifeguards and Artillery as part of John Nash's original design for Regent's Park. It is almost never in the news, despite being so close to the centre of London and busy Camden.

Ham & High: Regents Park BarracksRegents Park Barracks (Image: Christian Broom, Museum of London)

There is a good reason for this: the Ministry of Defence allows no access to the site for anyone except the military. Other military installations are occasionally open to the media, but not this one.

English Heritage provides a useful description of the buildings. Nash had originally intended the barracks to be situated in the northern area of Regent’s Park, well away from the residential area and separated from the rest of the park by Regent's Canal. However, Nash's plan was not accepted in its entirety by the Crown. One of the changes involving a change in the location of the barracks to its present site.

Originally designed to house 450 officers and men and 400 horses the barracks were almost entirely rebuilt in 1891-93. The rebuilding followed the original general layout, and carried out under the supervision of Colonel R Athorpe, Commanding Royal Engineer Home District.

The layout comprises a complex of buildings arranged around a parade ground.

Other buildings at the northern end of the site include the Gothic chapel which was built in 1857 and the hospital which was built in 1877. Three parallel blocks used for soldiers accommodation and stables, service buildings and the riding school were all built in 1891.

The officers' mess at Regent's Park Barracks is the only building of the original development to survive the 1891-93 rebuilding of the barracks. Originally known as the "officers' house", it is located on the east side of the parade ground, backing onto the canal.

Ham & High: New year celebrations in 1908 at Regent's Park BarracksNew year celebrations in 1908 at Regent's Park Barracks (Image: Contributed)

So why is the military so determined to keep the barracks out of the public eye?

The explanation can be found in the most public hostage siege in recent British history.

On April 30, 1980 six armed Iranians stormed the Iranian embassy in South Kensington.

The gunmen took 26 people hostage and held them at gunpoint. On May 5, in the full glare of the television cameras, elite troops of the Special Air Services (SAS) stormed the building, abseiling down from the roof. The operation lasted just 17 minutes. All but one of the terrorists were killed.

Ham & High: Members of the SAS on the first floor balcony at the Iranian Embassy in London when two explosions ended the six day siege at the building in Princes GateMembers of the SAS on the first floor balcony at the Iranian Embassy in London when two explosions ended the six day siege at the building in Princes Gate (Image: PA Archive/PA Images)

In the aftermath of the raid the Daily Mail reported that the SAS soldiers were visited by the then prime minister, Margaret Thatcher.

'Returning to their temporary London quarters at Regent’s Park Barracks, they were soon knocking back cans of cold beer when they were called to attention for a special visitor. Mrs Thatcher moving among them with handshakes and congratulations.

"Makes us proud to be British," she told them.'

Mrs Thatcher’s husband, Denis, is reported to have spoken to one of the SAS soldiers, known as "Tom" at a later date.

“He had a big grin on his face,” ‘Tom’ explained.

“You let one of the bastards live,” said Denis.

“We failed in that respect,” Tom replied.

Tom was later awarded the Queen’s Gallantry Medal.

It was only when the medal was sold at auction by Bonhams in September 2010 that his true identity was revealed. Tom was Sergeant Thomas GC Palmer. His medal was sold with the jacket and trousers he wore during the Iranian embassy siege, complete with SAS insignia, and a photograph of his visit to Buckingham Palace in June 1981, to receive the medal from the Queen.

Regent's Park Barracks is the London headquarters of at least one regiment of the SAS. This is what was once called the Artists Rifles, but this unit was disbanded in 1945. It became the 21 Special Air Service Regiment (Artists) (Reserve). Its responsibilities are said to include counter revolutionary warfare and counter-terrorism, as well as close protection.

There are vague hints of other activities in the Regent’s Park Barracks, but that is all. No wonder it is off limits.

Ham & High: A Regent's Park Barracks paradeA Regent's Park Barracks parade (Image: Contributed)

Locals remember the barracks in friendlier days, when it was home to the Royal Horse Guards. Children were once allowed to go in and care for the horses. Troops of horses, with riders mounted at strategic intervals, would canter onto our streets for exercise in the early mornings.

But the horses have gone and only the mysterious barracks remain.

Martin Plaut is the co-author with Andrew Whitehead of Curious Camden Town and Curious Kentish Town.

Ham & High: A Regent's Park Barracks paradeA Regent's Park Barracks parade (Image: Contributed)