The fiesta de Gracia also had amazing Castells (Castles), traditional Catalan human towers. We only brought our small point-and-shoot camera, but we made movies out of the images so that you could get an idea of the magnificence of the building of the Castells by the Castellers. However, these movies do not capture the solemn beauty of the castle building and we highly recommend that if you ever have a chance to see these artists in person, it is well worth it.
Wikipedia has a great explanation of the event:
A castell is considered a success when stages of its assembling and disassembling, can be done in complete succession. The assembly is complete once all castellers have climbed into their designated places, and the enxaneta climbs into place at the top and raises one hand with four fingers erect, in a gesture said to symbolize the stripes of the Catalan flag. The enxaneta then climbs down the other side of the castell, after which the remaining levels of castellers descend in highest-to-lowest order until all have reached safety.
Aside from the people who climb to form the upper parts of the tower, others are needed to form the pinya, or bottom base of thecastell, to sustain its weight. Members of the pinya (most often men) also act as a 'safety net' if the tower structure collapses, cushioning the fall of people from the upper levels.
The castell is built in two phases. First, the pinya the base of the tower is formed. People forming higher levels of the tower move to a position from which they can easily get to their place in the tower. This is done slowly and carefully, and as subsequent base levels are completed the castellers in the pinya determine if their base is solid enough for construction to continue. Then, when the signal to proceed is given, bands begin to play the traditional Toc de Castells music as a hush comes over spectators of the event. The upper layers of the tower are built as quickly as possible in order to put minimal strain on the lower castellers, who bear most of the weight of the castell. The disassembly of the castell, done amidst the cheering of the crowd, is often the most treacherous stage of the event.