The hand at the top

It is festival time in Barcelona: at La Mercè, the square in front of the town hall — Plaça de Sant Jaume — holds a diada castellera on the nearest Sunday, and on 24 September, the Virgin's own feast day, the local colles perform.

I won't tell you which year you're in. It doesn't matter: this is the Sunday the tower goes up.

You look up and see the tower. You're wrong: the story starts down here, in the pinya — the throng that holds the base, which UNESCO says anyone is welcome to join.

Level 1

The tower runs six to ten storeys high. From the second level up — the tronc — each storey carries two to five people, standing on the shoulders below. The heaviest stand low; the lightest, high.

You hear it before you see it: the gralla, the shrill pipe from the oboe family, setting the rhythm the tower is built to. The toc de castells is played by at least one gralla and one timbal.

Climb on: the shoulders beneath you thin out with every storey, and the square sounds further below. (The gralla you hear is a simplified electronic rendering from a public-domain score; the timbal beat and the crowd are illustrative — read the method at the end.)

Halfway up the tronc, the tower starts to sway. You feel it in your ankles before you see it: a slow motion, like a boat.

The sway drawn here is a stylisation — the levels are documented, the motion is for feeling.

And then the strange thing happens: you stop holding on. Hands you have never seen hold you — and you start to trust them.

The top three storeys — the pom de dalt — are climbed by the smallest and lightest: children and youths, girls and boys.

Everyone up there wears a helmet. Since 24 March 2012, when the federation assembly at Molins de Rei made it mandatory for the enxaneta, the acotxador and the dosos, at any age, in rehearsal and in the square, in all castells except the pilar — and the rule stands today.

Right at the top climbs the enxaneta — usually a light, agile young girl. Reaching the summit, she raises one hand: l'aleta.

At exactly that moment the gralla breaks into the fragment called the toc d'aleta: the sign that the tower has been carregat — crowned. No raised hand, no aleta.

But wait: if you climbed all this way for the view, you climbed in vain. The point was never what you can see from the top.

You climb down through shoulders and only now do you count them: the dozens of people in the pinya you never saw — they held you the whole time.

Down below there is room for you. “Making the pinya” — fer pinya — has become the Catalan synonym for working together. A colla has between 75 and 500 members, and the tower season runs from April to November.

Since 2010, human towers have been on UNESCO's Representative List of intangible heritage — decision 5.COM 6.40, taken in Nairobi: Catalans recognise them as “an integral part of their cultural identity”. They have gone up without a break for over 200 years, and the colles grew from 6 in 1970 to 56 in 2009.

At the opening of the 1992 Olympic Games, 12 towers went up at once: 2,174 people from 16 colles. In 2008, 37,418 people voted them among the treasures of the heritage.

The crowning is only half of it: a tower fully counts — descarregat — only if it comes down in order, the way it went up. Crowned but fallen on the way down — carregat — ranks below.

And the second surprise: the crowd below makes the top, and anyone may join it — even senior citizens. The earliest records of the practice come from near Tarragona, in the early 18th century.