Famous for its tradition of building human towers or 'castells', the La Merce Festival honors Barcelona’s patron saint and is held annually in Spain. The custom 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. A 'castell' is considered completely successful when it is loaded and unloaded without falling apart. During the week-long festival, close to two million people attend cultural and artistic presentations held throughout the city. In 2010, 'castells' were declared by UNESCO as one of the Masterpieces of the Oral and Intangible Heritage of Humanity