In pictures: Spain's human tower competition

Thousands of people come together annually in Spain to participate in the tallest human tower contest. The 'Concurs de Castells' is an event that sees teams of up to 500 people form impressive towers that are up to 10 tiers high.

The tradition dates back to the 18th century and is believed to originate from the human towers built by dance groups. Each team, which includes children, carefully builds its towers with their strongest and heaviest members interlocking their arms at the bottom. Meanwhile, the lightest make a dangerous ascent to the top.

Teams compete in over five rounds, with points being awarded based on the difficulty and height of each tier. The competition comes with danger however, as some suffer injuries when the towers inevitably topple over. Around 20,000 spectators come from across the globe to witness the spectacle, which is listed on UNESCO's cultural heritage list.