London layers two millennia of history beneath a restless global present: Roman walls, Tudor palaces and Victorian viaducts share streets with 21st-century glass shards that pierce a famously gray sky. Crown jewels sparkle in the Tower while traders shout in 300 languages at Borough Market; at dusk, pubs glow like lanterns along the Thames, whose curves host everything from Shakespeare’s Globe to the neon South Bank. A magnet for 11 million residents—and legions more visitors—it is both royal seat and open-air museum, stitched together by an Underground that first proved cities could move underground.