The castellers, or traditional human castle-builders, of Catalonia are an incredible sight to behold: Using only the bodies of their teammates, these groups of men, women and children climb quickly and intently upwards -- grasping hips with hands, digging bare feet into the crooks of knees and folds of cummerbunds, hoisting their full weight up to stand on the shoulders of the person above them -- to construct soaring, heart-stopping human towers of nine to 10 levels high. They perform from June to November in various towns and cities of Catalonia, including Barcelona.