These amazing photos of people constructing complex and terrifying human towers were taken at the 26th Concurs de Castells in Tarragona in the Catalonia region of Spain. The building matter used in the construction of each tower is people – up to 500 of them, working together in a tightly organised system.

Castells human towers Catalonia Spain
Colla Vella dels Xiquets de Valls complete their human towerLluis Gene/AFP

Each tower is carefully planned and engineered, with members positioned according to weight and strength, so there are no weak links. The teams use different building techniques, resulting in complex patterns when viewed from above.

The strongest and heaviest members interlock their arms to form a stable base, and then most of the other team members form human buttresses radiating out from each tower, supporting and strengthening it. They also act as a safety net, cushioning the fall of people from the upper levels if the tower collapses.

Castells human towers Catalonia Spain
Colla Jove Xiquets de Tarragona start to form a human towerAlbert Gea/Reuters
Castells human towers Catalonia Spain
Castellers de Vilafranca start to form a human towerAlbert Gea/Reuters
Castells human towers Catalonia Spain
Colla Vella de Valls start to build a human towerDavid Ramos/Getty Images
Castells human towers Catalonia Spain
Colla Vella dels Xiquets de Valls start to form a human towerLluis Gene/AFP

Then comes the hair-raising bit: constructing up to nine storeys, saving the lightest and youngest members for the upper levels. The most dangerous job of climbing to the pinnacle of the tower, saluting the crowd and then scrambling back down before the whole thing collapses is given to a girl as young as five. Accidents are rare, but there have been fatalities. In 2006 at a competition in Mataro, a 10-year-old girl fell to her death. The youngest members now wear foam-padded helmets.

Castells human towers Catalonia Spain
Members of the Colla Vella dels Xiquets de Valls build the next level of their human towerLluis Gene/AFP
Castells human towers Catalonia Spain
Younger member of the Colla Joves Xiquets de Valls climb up to form the tower's upper levelsAlbert Gea/Reuters
Castells human towers Catalonia Spain
Colla Vella dels Xiquets de Valls complete their human towerLluis Gene/AFP
Castells human towers Catalonia Spain
A supporter applauds as Colla Vella Xiquets de Valls complete a human towerAlbert Gea/Reuters
Castells human towers Catalonia Spain
Colla Joves Xiquets de Valls fall down after forming a human towerAlbert Gea/Reuters
Castells human towers Catalonia Spain
Members of the Colla Vella de Valls help a team mate who collapsedDavid Ramos/Getty Images
Castells human towers Catalonia Spain
Members of the Colla Xiquets de Reus support the human towerDavid Ramos/Getty Images
Castells human towers Catalonia Spain
Members of the Colla Xiquets de Hanghzou, who travelled all the way from China, support their team mates as they climb up to form the next level of a human towerDavid Ramos/Getty Images
Castells human towers Catalonia Spain
A member of Xiquets de Hanghzou takes pictures as the team builds a human towerDavid Ramos/Getty Images
Castells human towers Catalonia Spain
A young member of Castellers de Sant Cugat climbs up to form an upper level of the towerDavid Ramos/Getty Images
Castells human towers Catalonia Spain
Colla Jove Xiquets de TarragonaAlbert Gea/Reuters
Castells human towers Catalonia Spain
Members of various Colles watch their team members build a human towerDavid Ramos/Getty Images
Castells human towers Catalonia Spain
Members of the Colla Sagals d'Osona react as a human tower is builtDavid Ramos/Getty Images

The teams compete over five rounds, with points awarded for height and complexity. This year's Concurs de Castells in Tarragona was contested by 32 teams from around Catalonia – and one from China. A tense final saw Vilafranca triumph for the eighth consecutive time.

Castells human towers Catalonia Spain
Castellers de Vilafranca start to form a human towerAlbert Gea/Reuters
Castells human towers Catalonia Spain
An aerial view of the arena as Castellers de Vilafranca form their human towerAlbert Gea/Reuters
Castells human towers Catalonia Spain
The Colla Castellers de Vilafranca construct a human towerDavid Ramos/Getty Images
Castells human towers Catalonia Spain
Castellers de Vilafranca complete their human towerAlbert Gea/Reuters
Castells human towers Catalonia Spain
As soon as Castellers de Vilafranca's tower is completed, it is allowed to collapse
Castells human towers Catalonia Spain
Castellers de Vilafranca celebrate after forming a human towerAlbert Gea/Reuters
Castells human towers Catalonia Spain
Members of Castellers de Vilafranca celebrate after forming their human towerLluis Gene/AFP
Castells human towers Catalonia Spain
Members of the Colla Castellers de Vilafranca celebrate after building a human towerDavid Ramos/Getty Images

Human towers are considered to be such an integral part of the Catalan identity that they are recognised on the Unesco Representative List of the Intangible Cultural Heritage of Humanity.