A home so lush that its current owner is Rothschilds banking company, which hosts diplomatic and corporate events here that make the world go round, is the only surviving London mansion with an intact 18th-century interior, and you can see tours of the ground floor and a portion of the first floor. The home was begun in 1756 as a love nest by Diana Spencer’s ancestors (and, by extension, the future king’s), and the lavish gilt and carved decor repeatedly invoke the symbols of fidelity and virility. War damage spooked the Spencer clan, who moved out in the 1920s, and since then, they’ve gradually transferred the most precious elements to their estate at Althorp, 193km (120 miles) north of the city, and replaced them with equally fantastic facsimiles—the fireplace library, for instance, took 4,000 hours to carve. The original Painted Room suite was once at the V&A. Groups are limited to 20, so arrive early to secure a spot.