The City is the capital's fast-beating financial heart, with a powerful architectural triumvirate at its epicenter: the Bank of England, the Royal Exchange, and Mansion House. The "Square Mile" also has historic currency, as the place where London began. St. Paul's Cathedral has been looking after Londoners' souls for hundreds of years, and the Tower of London—a royal fortress, prison, and jewel house surrounded by a moat—has occasionally taken care of their heads.

The City is a dizzying juxtaposition of the old and the new. You’ll find yourself immersed in historic London if you begin your explorations on Fleet Street, the site of England's first printing press and the undisputed seat of British journalism until the 1980s. Nestled behind Fleet Street is Dr. Johnson's House, former home of the author of A Dictionary of the English Language, who claimed that "when a man is tired of London, he is tired of life." The nearby church of St. Bride's, recognizable by its tiered-wedding-cake steeple, is a Sir Christopher Wren gem and still the church for journalists, while eastward rises the iconic St. Paul's Cathedral, also designed by Wren and largely considered the architect’s masterpiece. You’ll encounter more of traditional London at the Central Criminal Court (nicknamed The Old Bailey, and home to London's most sensational criminal trials) and the 800-year-old Smithfield Market, whose Victorian halls are the site of a daily early-morning meat market. Nearby are the ancient church of St. Bartholomew the Great and St. Bartholomew Hospital, both begun in 1123; the Guildhall, the site of the only Roman amphitheater in London; the church of St. Mary-le-Bow; and the maze of charmingly old-fashioned, narrow streets around Bow Lane.

You can put all this history into context at the Museum of London, where archaeological displays include a portion of the original Roman Wall that ringed The City.

Just beyond rises the modern Barbican Centre, a concrete complex of arts venues and apartments that was controversial at the time it was built, but now has become an indispensable part of the London landscape. The sight of several new structures rising above The City—most famously the Lloyd's of London Building and the Swiss Re Tower, popularly known as "the Gherkin"—may or may not be more reassuring.

The Monument, near the banks of the Thames, was built to commemorate the Great Fire of London of 1666. From here, the river leads to one of London's most absorbing and bloody attractions, the Tower of London. Tower Bridge is a suitably giddying finale to an exploration of this fascinating part of London.

There is another reason that makes The City such an intriguing place to visit at the moment: with the constant building of new skyscrapers in the financial district, the ancient skyline is changing at a rapid pace. Cross the Thames anywhere from Waterloo to Tower Bridge and half the people you’ll see taking in the view are probably Londoners, paused to wonder in astonishment as yet another glass-and-steel monolith seems to have popped up since last week. You could leave years between your visits, or merely months; either way The City will never be the same as when you saw it last.