The description from this video on its Vimeo page pretty well sums it up. I’ve never seen anything quite like it:
In the city of Tarragona, Spain, castellers gather every two years to see who can build the highest, most intricate human castles. This Catalan tradition requires astonishing strength, finesse, and balance. Not to mention courage.
I’d add that a little bit of crazy goes along with the courage. At least the children wear helmets.