Imposing, ornate, and adored by music lovers worldwide, it’s one of the few performance arenas on earth where, once they have performed there, artists can truly claim to have made it. During the summer, the storied BBC Promenade Concerts (the Proms) fill this historic, 5,200-seat circular hall with classical music, but the rest of the year, the space books a hodgepodge of tours, arena-style musicals, Cirque du Soleil, concerts (the house organ is 21m/69 ft. tall and has 10,000 pipes)—even tennis matches.