Teamwork is, of course, crucial in many sports. But few require as much co-operation as the building of a “castell”. Taking its name from the Catalan word for castle, the activity involves the construction of a human tower—the best groups can stack up ten storeys—in a competitive tradition that began in the 18th century. Catalonia’s strongest squads will competing in Barcelona this weekend at La Mercè festival. Beefier builders form a wide base, which also acts as a crash mat. The middle layers comprise several people in a circle, arms locked; then, helmeted youngsters clamber up to the top, urged on by drumming and piping. Getting up is only half the challenge. Many towers collapse on the way down, and ambulances often wait beside the onlookers. With hundreds of members, the teams will be well-rehearsed: a fumbled pass is a nuisance, but a tumbling castell is a disaster.