In the city of Tarragona, Spain, castellers gather every two years to see who can build the highest, most intricate human castles. Pretty intense place, Catalonia. The tradition of building human towers originated in Valls, near the city of Tarragona, in the southern part of Catalonia towards the end of the 18th century. Later it developed a following in other regions of Catalonia even Majorca and, after prohibition under Franco, currently has become very popular in parts of Spain. However, the best and most skilled castellers are still found near Tarragona.
Check Mike Randolph's pics (like this one) - amaaaazing! He also took this video:
The motto of castellers is "Força, equilibri, valor i seny" (Strength, balance, courage and reason).
* Strength: A casteller is usually a stocky person. The first castellers were peasants that were accustomed to holding great weights and were under much physical exertion. * Balance: For supporting other above in the castell while relying on those below for support requires a strong sense of balance and trust. * Courage: The most important characteristic for castellers, especially for young children forming the highest levels of the castell. * Reason: Rehearsing and performing requires a great deal of planning and reasoning. Any error can cause the structure fail and break apart.
There's a good explanation of the different types (with pics) here.