TARRAGONA, Spain - People watched in amazement during the 26th Human Tower competition in Spain Sunday.

The tradition of building human towers or “castells” dates back to the 18th century.

It takes place during festivals in Catalonia, where teams compete to build the tallest and most complicated towers. The structure of the human towers varies depending on their complexity.

A castell is considered completely successful when it’s loaded and unloaded without falling apart.

The highest human tower in history was a 10-floor structure with three people in each floor.

In 2010, castells were declared one of the masterpieces of the oral and intangible heritage of humanity by the United Nations Educational, Scientific and Cultural Organization.