The Catalan tradition of building human towers or castells (the Catalan word for 'castle') originated in Valls, near the city of Tarragona in Catalonia, an autonomous community of Spain with its own distinct language and culture. Since the 1980s the phenomenon has become increasingly popular and widespread, and colles (or 'clubs') have now been established throughout Catalonia.

 In 2010, the castells were classified by UNESCO as one of the Masterpieces of the Oral and Intangible Heritage of Humanity.

So what is the secret underpinning these swaying spectacles? Those forming the base of the tower – the pinya – are the first to take their positions. As the function of the pinya is to sustain the weight of the tower (and to act as a ‘safety net’ in the event of upper tiers collapsing), this is usually composed of a huddle of heavyset men. When the base layers are deemed sturdy enough, the gralla (a traditional Catalan reed instrument) begins to sound, signalling the various stages of construction, and the people forming the upper tiers then swiftly shimmy up the base layers using their fellow members’ waist sashes (faixes) for leverage. Various formations are possible depending on the number and distribution of people, but the one unchanging feature is the enxaneta, the child who scrambles to the summit and raises one hand with four fingers extended – a salute which is said to represent the four stripes of the Catalan flag – to indicate that construction is complete. De-construction occurs layer by layer, starting with the highest tiers, and is often the most difficult and dangerous part of the display.