A "castell" is a tower of humans climbing up upon one another

This gigantic tower of humans is fantastic. Known as a "castell", these human towers have been created during festivals in Catalonia for over 200 years. 

I can't imagine the bravery it must take to be the person at the very top of the castell. You better hope that nobody below you missteps, or has an itch they need to scratch. 

On the other hand, it looks like great fun to participate in this tradition. The performers are volunteers who practice multiple times a week. I hope to see a castell in person some day.