Time Out says
Among the highlights of traditional activities during the Gràcia Festival is the Casteller Day. Castellers are human tower builders, and on Saturday August 17, you can watch groups from Reus (the Xiquets), Vilafranca, Valls (youth group the Xiquets), and the locals from Vila de Gràcia itself. The organisation of Castellers invites everyone to participate by forming part of the 'pinya' (literally 'pineapple'), which is the group of people who huddle together around the castellers to give them support in every sense of the word. You're urged to use caution should you decide to take part.