The old St Paul’s, with its magnificent spire, stood on this site for 600 years before it was claimed by 1666’s Great Fire. It was so beloved that when Sir Christopher Wren was commissioned to rebuild London’s greatest house of worship, he tried to outdo the original, devoting 40 years to the project and going one further by crowning it with a mighty dome—highly unusual for the time. It was also stark and plain, like the Catholics did their churches, not Gothic and complicated like the Protestants. My, how people talked.

St Paul’s cost £750,000 to build, an astronomical sum in 1697 when the first section opened for worship, and now, it costs £3 million a year to run. Wren overspent so badly that decoration was curtailed; the Byzantine-style mosaics over the Quire weren’t added until Queen Victoria thought the place needed spiffing up (and some purists are still complaining). Stained glass is still missing, but that just allows the sweep and arch of Wren’s design to shine cleanly through. Many foreigners were introduced to the sanctuary during the wedding of Prince Charles and Lady Diana Spencer in 1981, but the pulpit also saw a sermon by Martin Luther King in 1964 and Churchill’s funeral the next year. Frustratingly, you’re not allowed to take photos on your tour.

The Great West Doors, largely unused unless you’re important, are 27m high (90 ft.) and on their original hinges; they’re so well-hung that even an infirm vicar can swing them open. In 2005, the Cathedral completed a £10.8-million cleaning program; a stone panel beside the doors was left filthy to show just how gloomy centuries of candles had made it in here. The High Altar has a canopy supported by single tree trunks that were hollowed out and carved, and its 15th-century crucifix and candlesticks require two men to lift. (They’re nailed down, anyway. As one docent, a half-century veteran of Cathedral tours, lamented, “You’d be surprised what people try to steal.”) Behind it is the American Memorial Chapel to the 28,000 American soldiers who died while based in England in World War II; in a glass case, one leaf of a 500-page book containing their names is turned each day. The organ, with 7,000 pipes, was regularly played by Mendelssohn and Handel, and the lectern is original. Give special admiration to the impossibly fine limewood carvings in the Quire by Grinling Gibbons—dark wood is original and lighter wood has been replaced or restored. Nearby, there’s a monument to prettyboy poet and cleric John Donne that still bears on its urn the scorch marks it suffered in Old St Paul’s during the Great Fire (it was the only thing that survived the conflagration).   

Eight central pillars support the entire weight of the wood-framed Dome; Wren filled them with loose rubble. In 1925, engineers broke them open to find the debris had settled to the bottom, and they filled them again with liquid concrete. If you’re fit, you can mount the 259 steps (each just an awkward 13cm/5 in. tall, but double-wide with some spots to catch your breath) to the circular Whispering Gallery, 30m/98 feet above the floor. Famously, its acoustics are so fine you can turn your head and mutter something that can be understood on the opposite side, 33m/108 feet away. That’s in theory; so many tourists are usually blabbing to each other that you won’t hear a thing, although it is a transcendent place to listen to choir rehearsal on a mid-afternoon. Climb higher (you’ve gone 378 steps now, and now the stairs get tight) to the Stone Gallery, an outdoor terrace just beneath the Dome. Catch your breath, if you choose, for the final 152-step push to the Golden Gallery—you’ll be scaling the inner skin of the Dome, past ancient oriel windows and along tight metal stairs. There are three domes, in fact—the inner skin with the monochrome paintings by Sir James Thornhill, the leaden outside, and in between, a brick one that holds it all together. That’s what you traverse—look sharp for ancient carved graffiti by tourists who preceded you in the 1700s. It’s safe, but it’s not for those with vertigo or claustrophobia. The spectacular 360-degree city view from the top (85m/279 ft. up), at the base of the Ball and Lantern (you can’t go up farther), is so beautiful it defies full appreciation. For more than 250 years, this was the tallest structure in a city that itself was atop the world.

If you miss the Crypt, you’ll have missed a lot. In addition to memorials to the famous dead (such as Florence Nightingale and plenty of obscure war heroes), you’ll find tombs, such as composer Sir Arthur Sullivan’s ebullient bronze plate and the tombs of two of Britain’s greatest military demigods: Admiral Horatio Nelson (whose body was preserved for the trip from the battlefield by soaking in brandy and wine), and Arthur, Duke of Wellington (flanked by flags captured on the field of battle; they will hang there until they disintegrate). To the right of the OBE Chapel, in Artists Corner, you’ll find the graves of the artists J. M. W. Turner and Henry Moore, plus, under a black slab by a window, Christopher Wren himself, who rests inside his masterpiece. “I build for eternity,” he once said, and so far, so good: In 2010, the cathedral celebrated 300 years since its completion.

Volunteers, called “supers,” lead free 90-min. tours at 10 and 11am, and 1 and 2pm, and half-hour highlights tours six times a day between 10:30am and 3pm. Listen closely, because they are the elder statesmen; many have been here for decades. Lest you forgot it’s actually a cathedral, you can also worship here outside of sightseeing hours—for free. If you’re hungry, scope out the cafe, one of the cheaper options in this neighborhood. When you’re done inside, head just east to the One New Change shopping complex, where the free rooftop terrace has some spectacular close-up views of the Dome—perfect for vacation snaps.