It was rush hour on a weekday and I was weaving my way down the Strand, one of central London’s most famous thoroughfares. The street hummed with tourists, students and lawyers. Double-decker buses rattled. Cyclists sweated. Black cabs swerved.

Just east of the where the Strand turns into Fleet Street, beyond the 19th-century legal bookshop of Wildy and Sons, stood a small stone archway. Compared to the imposing structure above it – a timber-framed, Jacobean townhouse – it was almost unnoticeable. I turned in.

Here, on tiny Inner Temple Lane, was a hidden world, one that was lovely, leafy and serene, overlooked by graceful Gothic and Victorian buildings and patchworked with gardens and miniature courtyards.

The area, known as Temple, remains far less known to tourists than other nearby attractions like St Paul’s Cathedral or Trafalgar Square. And most of those who do find their way here don’t realise Temple’s biggest secret: this whole area was once the stronghold of the Knights Templar.

The medieval order, known for their role in the Crusades and as one of the Middle Ages’ most powerful and wealthy religious orders, lived, prayed and worked here from about 1185 up until their dissolution in 1312.

They built monastic dormitories, chambers and two dining halls – now known as Middle Temple Hall and Inner Temple Hall, though they’ve been rebuilt many times over the years – and, most famously, Temple Church.

“They lived right here,” said Robin Griffith-Jones, the reverend of Temple Church and a historian of the Knights Templar. (In a sign of how historic and traditional this area is, his official title is Reverend and Valiant Master of the Temple). “The hall of the Templars was what is Inner Temple hall now – right over there. And the priest’s house was where my house is.”

In 1120, Christian knights had just captured Jerusalem in the First Crusade. But even while the holy city was safe, the pilgrimage routes to get there were not. Travellers were routinely attacked, robbed and even killed.

A handful of knights took monastic vows and devoted themselves to protecting the pilgrims and their routes. In return, the king of Jerusalem gave them headquarters on the Temple Mount. The Knights Templar was born and they were soon world-renowned for their courage.

“They were a very disciplined fighting force – and hugely self-sacrificial. If there was a disaster in battle, they were decimated. They didn’t run away. They just got killed,” said Griffith-Jones.

They also became extraordinarily rich. As well as owning land and other assets, they didn’t have to pay tithes. They were also the first to issue what today we would call cheques. If a pilgrim was leaving home, they could give the Templars all the money they’d want in the Holy Land, get a promissory note in return and collect that amount when they arrived. By 1191, they were so wealthy they were able to buy the island of Cyprus.

Little surprise then that by the mid-12th Century they needed a grander headquarters for their London chapter. By 1185, they had built Temple Church.

Today Temple Church doesn’t seem that grand, particularly when compared to nearby St Paul’s or Westminster Abbey. The surrounding buildings dwarf it, making its dome invisible from just a short distance. The circular nave in the west, which was built first, is just 17m in diameter. There is no elaborate gold gilding, no side chapels, no mosaic or paintings.

But as a round church modelled after the Holy Sepulchre of Jerusalem (there are only three others in Britain), Temple Church had one of the grandest claims of them all: to those in the Middle Ages, walking through it was the closest you could get to Jerusalem without actually undertaking the dangerous pilgrimage to get there.

Inside, the round nave has the fortress-like walls, small windows and heavy, pointed arches of the early Gothic. Effigies of some of the knights – including William Marshal of Pembroke, without whom England’s Magna Carta may not exist – lie grasping their swords in the stone.

The ‘new’ chancel, built 65 years later, extended the church east, this time with all the hallmarks of the fully-flowered Gothic style: thin, graceful columns, wide-span arches and huge windows that flooded the interior with light.

In the time of the Knights Templar, the painted walls and metal-plated ceiling would have shimmered in the candlelight. The floor was tiled. There were probably banners down the columns. And the windows, now mostly plain, may have been made of stained glass.

It was in that lovely, light-filled environment that the English order of the Knights Templar would meet and worship. It was also here that they would be initiated into the order. According to charges levied against them in 1307, when King Philip IV ordered the arrest of the Templars in France, the initiation rites included spitting on the cross, denying Christ, and kissing each other on the mouth, belly button and base of the spine.

By that point, the knights were no longer needed as crusaders. Their military stronghold of Acre, in present-day Israel, had fallen in 1291. The knights were still engaging in smaller-scale raids, but the Crusades had effectively ended – and, for the Church, had not ended well.

As well as no longer having any military purpose, the Knights Templars’ wealth had made them potential enemies of some powerful people – including King Philip IV, who owed them a vast sum of money.

The charges of devil-worship in their initiation rites quickly followed. Scores of knights were arrested on Friday 13 October 1307, and those who wouldn’t confess were burned at the stake. The rest scattered. In 1312, the order was dissolved.

The land at Temple went to the Knights Hospitaller, another military religious order. That order leased the land to lawyers in 1346, and today the Temple area is well known to England’s barristers, all of whom must belong to one of London’s four Inns of Court – medieval legal associations – in order to practice. Two of these Inns of Court, the Inner Temple and Middle Temple, are based here.

The Inner Temple still has a section of its medieval hall, complete with 15th-century fireplace. And with its hammer-beam ceiling and rich oil paintings, Middle Temple’s hall is largely what it would have looked like when it was built under Queen Elizabeth in 1562.

Today, though, you’ll see London’s barristers walking through the courtyards with small, rolling suitcases – the preferred method for transporting the stiff horsehair wigs they have to don at court. And visitors can usually peek in over lunch hours, or – better – advance book a tour of the Inner Temple.

Until 13 years ago, hardly any tourists came to this area at all. “It was the classic hidden gem,” said Griffith-Jones. “Part of its joy is that it is really tucked away: it’s as if you go into a secret garden as soon as you come in from Fleet Street. It is absolutely, ravishingly beautiful. And it was a disappointment to us that London’s residents, the people who work here, the people who visit it, so few knew about it.”

But then a certain novel was published.

“One Monday morning [in 2003], there was a queue of young Americans standing outside the door,” Griffith-Jones said. “The verger opens up and they ask him, ‘Have you read the book?’ And of course the verger thinks they’re talking about the Bible.”

Instead, they were talking about Dan Brown’s Da Vinci Code. It would turn out to be one of the most popular novels of the 21st Century – and one of its main scenes was set at Temple Church.

In those boom years, the church saw around 500 visitors every day.

However, those days seem to be over. When I was there, there were just two families and a couple wandering through the space.

Today, Temple Church – and Temple – are back to feeling like a hidden world in the heart of London, one that’s serene and rich with secrets.

And that, in many ways, feels how it should be.

BBC Travel’s Mysteries of the Knights Templar tracks down the secret sites and mysterious ceremonies of what was once one of the most powerful and wealthy organisations in Europe.

This story is a part of BBC Britain – a series focused on exploring this extraordinary island, one story at a time. Readers outside of the UK can see every BBC Britain story by heading to the Britain homepage; you also can see our latest stories by following us on Facebook and Twitter.

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Correction: An earlier version of this article incorrectly stated that Acre was located in modern-day Syria. It is in Israel, near the borders of Lebanon and Syria. This has been fixed.