Members of the Castellers Collas Jove Xiquets de Tarragona fall as they tried to complete their human tower during the 25th Human Tower Competition in Tarragona, Spain, on Sunday, Oct. 5, 2014. (AP Photo/Emilio Morenatti)

The tradition of building human towers or “castells” dates back to the 18th century and takes place during festivals in Catalonia, where “colles” or teams compete to build the tallest and most complicated towers. The structure of the “castells” varies depending on their complexity.

A “castell” is considered completely successful when it is loaded and unloaded without falling apart. The highest “castell” in history was a 10 floor structure with 3 people in each floor. In 2010 “castells” were declared by UNESCO one of the Masterpieces of the Oral and Intangible Heritage of Humanity. (AP)

Castellers de Vilafranca form a human tower, called “castell”, during a biannual competition in Tarragona city October 5, 2014. The formation of human towers is a tradition in the area of Catalonia. (REUTERS/Albert Gea)