A dragon rears up bang in the middle of the road, wings outstretched, marking an invisible boundary. She (or he — with a dragon, how can you tell?) clutches a shield with scaly claws and snarls silently westward. The dragon is only a statue, perched on a fancy marble base with a relief of Queen Victoria holding a threatening gold scepter, but stands at the edge of the City of London, the original Londinium, the capital founded nearly two millennia ago by the Romans. West of the dragon lies the London everybody knows: Buckingham Palace, Piccadilly Circus, Harrods, the Houses of Parliament. To the east, it’s a world financial center, with glittering skyscrapers proclaiming the names of banks, brokerages and law firms. But behind the shiny glass, concrete and steel, the London of 800 years ago, even 1,800 years ago, is still there, hiding in plain sight: a London of quasi-secret gardens, ancient stone fortifications cheek-by-jowl with digital start-ups, Roman ruins lurking in parking garages and forgotten churches where lie the bones of 14th-century queens.

Officially known as the Temple Bar monument, the dragon is my signal to get off the bus. From the early Middle Ages until 1878, there was a physical gate here, a bar, controlling who went in and out of the City, the square mile of Ur-metropolis that maintains its own mayor and its own system of governance. The “Temple” part of the name comes from the Knights Templar, who acquired property here in the 1160s.

The No. 23 chugs away, leaving me on Fleet Street in front of the Victorian crazy castle that houses the Royal Courts of Justice — this whole area is a hotbed of lawyers. I cross the road and slip down a narrow passage, just wide enough for two (skinny) people, and emerge in a courtyard bright with sunny daffodils just beginning to open. All of a sudden, there’s an almost unnatural silence, as if a soundproof curtain has fallen, shutting off the grinding, shouting and striving of the 21st century.

The Middle Temple and the Inner Temple, the name given to the collection of libraries, lawns, dining halls, quadrangles and chambers (law offices), some dating from the time of Elizabeth I, are two of the four Inns of Court, the British equivalent of the bar. But before the attorneys took over, the Templars, officially “the Poor Fellow-Soldiers of Christ and of the Temple of Solomon,” ruled this part of the City.

Despite the swivel-eyed conspiracy theories of “The Da Vinci Code,” the Templars weren’t a sinister bunch of church enforcers hiding the marriage of Jesus and Mary of Magdala; they were an elite transnational fighting force of Crusaders with one eye on conquering the Middle East in the name of Jesus and the other on amassing money and power. In 1307, having failed to retake the Holy Land but having succeeded too well at getting rich, the Templars were accused by the King of France of paganism, homosexuality and spitting on the cross. The real story was that the king owed them a lot of money. The order was disbanded, and many of the knights were burned at the stake.

Happily, not all traces of the Templars are gone: Their headquarters, round as a crown and consecrated in 1185, still stands here amid the stone walks and green grass. The church was supposedly modeled on the Holy Sepulchre, which features the tomb of Jesus, in Jerusalem. Henry III added a rectangular chancel in 1240. It’s one of the calmest, least-touristed places in all of London. Yet as I stand in the circular nave, surrounded by a ring of dark Purbeck marble columns, I feel as though I’m being watched. And I am — sort of — by all these carved stone faces, each with what you might call an irreverent, maybe downright rude, expression, sticking out their tongues, crossing their eyes, glaring, sneering, looking like a panel of political pundits on TV. In contrast, the nine effigies of 12th-century knights on the floor are dignified and serious, though they do look a bit like a medieval slumber party.

I emerge into rare spring sunshine and stroll around the garden, three acres of plane trees and flower beds. Shakespeare’s “Twelfth Night” was first performed in Middle Temple Hall in 1602; he also set a pivotal scene of “Henry VI, Part I” here in the Temple garden, where the great lords of England declare allegiance to rival royal heirs by dramatically picking a white rose for York or a red one for Lancaster, kicking off the Wars of the Roses. It’s complete fiction, sadly; nevertheless, in a few weeks, there will be red and white roses blooming here. At the moment, the garden is full of gem-colored tulips, sharp-scented narcissus and barristers in sharp suits going to and from court, pulling small, wheeled carry-on bags that hold their briefs, gowns and horsehair wigs.

I walk down Fleet Street, which takes its name from the River Fleet, one of London’s now-underground streams, up Ludgate Hill, supposedly named for Lud, a mythic king of the Britons (but really from an Anglo-Saxon word for “swing gate”), past the pigeon-gray dome of St. Paul’s and the penitentiary architecture of the London Stock Exchange, arriving even further back in the past than the Temple.

To see what’s left of Londinium, the greatest city of Roman Britannia, you must go underground. Literally. Beneath a 1960s office block on Lower Thames Street, there’s a Roman bathhouse complete with central heating, part of a 2nd-century riverside villa. “Bread and circuses,” the poet Juvenal’s depiction of Roman hunger for vicious entertainment, haunt the basement of the Guildhall Art Gallery, where, in 1988, builders discovered the remains of an amphitheater dating from A.D. 70. An eerie digital display reminds you that the arena held 6,000 spectators and that countless gladiators (and animals) fought and died here for the entertainment of the masses. Most of the structure is gone, but one of the drains remains, as does the stone foundation and the sand on the ground used to soak up the blood.

The Romans were into amphitheaters, but they liked walls even better: Emperor Hadrian had one built across the top of England in 122; in 142, they built the Antonine Wall in Scotland, both to ward off “barbarians.” After Queen Boudicca and her Iceni tribesmen burned Londinium to the ground in about A.D. 61, the Roman governor figured his city needed a wall, too, though it took more than 100 years to finish.

The Temple Bar monument acts as a makeshift signpost on the edge of the City of London, the original Londinium. Modern highlights are to the east and west. (John McKenna/Alamy Stock Photo)

St. Giles’ Cripplegate survived the Great Fire of 1666. (Glyn Thomas Photography/Alamy Stock Photo)

As the centuries passed, some of the wall fell, some were cannibalized for stone, and much was covered up as the ground level rose. War and archaeology eventually revealed the Romans’ impressive handiwork. In 1940, German bombs devastated much of the City, but in the rubble they found long stretches of Roman wall as well as a Roman fort built circa A.D. 200. Below the road, called London Wall, I walk across the bridge over a little lake to the lovely, 700-year-old St. Giles’ Cripplegate, one of the few City churches to survive the Great Fire of 1666. The name comes from “cruplegate,” the Anglo-Saxon term for tunnel. The church — burial place of the poet John Milton, who declared London “a city of refuge, the mansion house of liberty, encompassed and surrounded with His protection” — was damaged by the same bomb that uncovered the Roman remains but has been restored to sit somewhat incongruously in the midst of modern concrete on three sides and a tiny garden on the fourth.

Actually, the garden, full of flowering trees and next to the splendidly eclectic Museum of London, is one of the best places to see the Roman wall. The lower sections are original; the higher elevations medieval. The Saxons rebuilt much of the wall beginning in the late 9th century as a defense against the Vikings, then Plantagenet princes added towers and fancier ramparts as a defense against other Plantagenet princes.

You can also see the remains of a Roman fort — a gate, a guardhouse and part of a tower — but only on a Museum of London guided tour (check the “Events” link on its website). The place where centurions paraded their cohorts and soldiers kept a lookout watching for rebellion-minded Celts and other threats is now in a locked area of an underground parking garage.

The Museum of London is a curiously underappreciated City jewel, rarely stuffed to the gills with schoolchildren and tourists, not as overwhelmingly vast as the British Museum, yet a treasure house of the ancient and the extraordinary, such as the 300,000-year-old skull of an auroch and the tooth of a woolly rhinoceros found less than a half-mile away, as well as Viking war axes and Lord Nelson’s jeweled sword. The Roman collection has more than 47,000 objects (including some leather “bikinis”), but the most impressive are the mosaic floors uncovered in the East End and the 4th-century lady found in nearby Spitalfields.

The noblewoman was buried in a decorated lead coffin with a scallop-shell pattern. Arabian embalming oils used on her demonstrate that Londinium was no hick town on the edge of the Empire but rather a cosmopolitan marketplace. The isotopes in her tooth enamel reveal that she was actually born in Rome, and the jet beads, rich damask and gold thread of her shroud suggest she was an aristocrat.

A full flask of wine in her sarcophagus could indicate that she worshipped Bacchus, too. The wine god’s local cult had installed itself in the 2nd-century Temple of Mithras, less than a mile away, when the soldiers’ god fell out of fashion. The Mithraeum was discovered at Walbrook (yet another hidden London stream that used to run under the Roman wall) in 1954 when an insurance company started digging the foundations for its new offices. The temple was moved and placed in storage; few people have had a chance to see it. But there’s hope that will change next year, when the temple is reinstalled on the lower level of Bloomberg’s new London headquarters, pretty much in the exact same place it was found. The museum has a wonderful sculpture dug up from the Mithraeum — a post-party Bacchus and Pan, drunk off their immortal butts, propped up between a satyr and a maenad.

After a cappuccino in the museum cafe, watching the encyclopedic 24-hour clock proclaiming random, yet compelling, London factoids (the first London coffeehouse opened in 1652; the value of the Crown Estates in London is 6 trillion pounds; London has 460,000 trees), I head out into a spritzy spring rain to visit some queens and homicidal women.

Christ Church Greyfriars was part of a Franciscan monastery, bankrolled in 1306 by Margaret, a wife of Edward I and built at Newgate, along the Roman wall. It was the second-largest church in medieval London, after St. Paul’s Cathedral, and the burial place of Queen Margaret, her daughter Queen Joan of Scotland and Queen Isabella, the beautiful and ruthless wife of Edward II. Along with her lover Roger Mortimer, Isabella, the “She-Wolf of France,” deposed her husband in 1326 and may have conspired in his death. “Le Morte d’Arthur” author Sir Thomas Malory was buried here, too, as well as Elizabeth Barton, a nun later known as the “Mad Maid of Kent,” who went around prophesying that Henry VIII would burn in hell if he married Anne Boleyn.

The monuments that marked where these notables lay are gone now, damaged first during Henry VIII’s seizure of church lands in 1538, then burned in the 1666 Great Fire. The church was rebuilt in 1704 by Sir Christopher Wren — who also designed the “new” St. Paul’s — only to be burned again on the night of Dec. 29, 1940, when the Luftwaffe devastated the City in the “Second Great Fire of London.” St. Paul’s was saved; a score of beautiful old churches were not. Yet Christ Church Greyfriars’s outer walls, with Wren’s elegant arched windows, remain, as does the church tower, now a private residence (if you’re looking for a London pied-à-terre, it was recently on the market for $5.7 million). The nave has been turned into an elegant, slightly melancholy garden laid out on Wren’s floor plan with bay trees, pink roses and blue irises.

I stand there sheltering under my cheap umbrella, deleting the ugly commercial edifices of Merrill Lynch, British Telecom and Goldman Sachs in my head, wondering whether the ghosts will come out with the twilight. Queen Isabella is said to walk here in silks and furs, holding the still-beating heart of the murdered king. She has some competition: Lady Hungerford, executed for her first husband’s 1518 murder, makes the occasional appearance as well, sometimes to argue with the queen over which one is better looking. Agnes Hungerford was hanged in 1523 on the banks of the Tyburn, that now-disappeared river, along with other criminals who weren’t fancy enough to merit Tower Hill.

The soft drizzle and the dying light helps roll away the 21st century, back 700 years ago when Christ Church looked over at the huge medieval spire of St. Paul’s piercing the heavens at the top of a nearby hill, then nearly two millennia ago when the Romans were making their great city on the banks of the Thames. London is itself like a cathedral built on a Roman temple that was built on a neolithic henge, layer upon layer of history, each generation making its mark on top of the other. There’s always something else to dig up. Yet London, always generous, reveals its secrets to those who bother to look a little.