Currently owned by Rothschilds banking company, which hosts diplomatic and corporate events here, this lush home is the only surviving London mansion with an intact 18th-century interior; tours let you see the ground floor and a portion of the first floor. The house was begun in 1756 as a love nest by Diana Spencer’s ancestors (and, by extension, the future king’s); its lavish gilt and carved decor repeatedly invoke the symbols of fidelity and virility. Great 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 library fireplace, 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.