For a city that lost four-and-a-half centuries ago (Florence won), Siena has a lot of pride. This is because Siena is beautiful, and because Siena is full of Italians.

The city is divided into 17 contradas, which are kind of like residential colleges, except much more medieval and even more intense. Twice a year, in July and August, the contradas compete in a bareback horse race called the Palio. (A banner, or “palio,” is what the winning contrada carries across the city after the race.) This may sound like the tourist trap to end all tourist traps. It isn’t: the people who live in Siena take this stuff very seriously.

It requires a special effort for me to begin to conceive of why the Palio stirs up the emotions it does. The jockeys are hired guns from Sardinia, not contrada devotees, and the horses don’t even come from Siena. Of course, don’t say any of this to the Sienese, or you might find yourself at the wrong end of a thoroughbred.

Horses are assigned to each of the 10 participating contradas a week before the race. A few minutes into this year’s event, two big groups of people came to a stop on the horse-schlepping slope that leads to the main square, and for a moment it looked like they were all swimming the crawl, very fast, in place. To me, this was simply a new and fascinating cheer. Then a teenage girl flopped over the fence between the slope and the square, bawling. It dawned on me: nobody was swimming. Everybody was punching each other.

Apparently, one contrada had made fun of another’s horse. If in Rome you should do as the Romans do, in Siena, you should steer away from ridiculing another man’s cavallo.