Thousands gathered at an old bull ring in Tarragona, a city in Spain's Catalonia region about 50 miles southwest of Barcelona, over the weekend for its annual castells competition, where teams made of up to hundreds of people collaborate to build human towers.

Building human towers, or castells, is an old Catalan tradition dating back over two hundred years. Each castell (a Catalan word for castle) is built by a team, called a colla, consisting of between 75 to 500 men and women. Young and light members form the top of a tower while heavier members form the base. Music plays as a team erects its tower, usually between six and ten levels high.