Twisting spirals of smoke rose from fires in Florence. Looking on from his refuge in the hills, the old art historian Bernard Berenson was reminded of red-tinged clouds in the paintings of Albrecht Altdorfer. It was August 1944, and the German army was blowing up bridges to impede the Allies’ steady advance from Rome through northern Italy. When the smoke cleared, five of the six bridges across the Arno River had been reduced to rubble; only the Ponte Vecchio had been spared. The heavy stone buildings at either end had been demolished and crammed with explosive mines, making the bridge impassable. The center of Florence was cut off.

Or so it seemed. But the Ponte Vecchio has an elevated passageway that the Germans had overlooked. Running along the top of the bridge’s shops, the so-called Vasari corridor connects the Uffizi museum with the Palazzo Pitti on the south bank of the Arno, a distance of almost half a mile. The Florentines didn’t forget, of course. Within hours, a telephone line had been uncoiled over the whole length of the darkened corridor, and the partisans inside the city were reporting to British Special Operations. As soon as the war was over, the secret of the Vasari corridor was immortalized in a prize-winning movie. In "Paisan," directed by Roberto Rossellini, an American nurse and an Italian partisan make the perilous crossing, passing through the moonlit galleries of the Uffizi with bated breath as they seek their loved ones in the city.

Ever since the Romans set out to conquer the Italian peninsula, a bridge has spanned the Arno at this place, hence its name, Ponte Vecchio—"the old bridge." The Etruscans planted the town of Fiesole on the cooler slopes of a nearby hill, but the Romans wanted to guard their bridge. Caesar established a city on the sunburnt plain, populated it with retired soldiers and named it "Colonia Florentia" ("Flourishing Colony"). The name proved prophetic for an outpost that blossomed into Florence, the cradle of the Italian Renaissance.

The three arches of the Ponte Vecchio were built in 1345, reputedly by a pupil of the great Giotto, the previous bridge having been swept away by a flood in 1333. Current scholarship tends to credit the design to a master mason, as opposed to a painter, but the point is, it was a remarkably good-looking bridge in a no-frills, Tuscan way. It was lined with shops, most of them butchers. Two hundred years passed, during which Florence declined from a vibrant republic to a pompous grand duchy. It took only a few years of suffering the stench of the meat before the Grand Duke Cosimo I de’ Medici approved a plan for a private corridor to connect his offices, the Uffizi, with his residence, the Palazzo Pitti.

The corridor was constructed in five months in 1565, and has always been known by the name of its architect, Giorgio Vasari, who was better known as a painter. The Ponte Vecchio we admire today was transformed by Vasari’s corridor, which has more merit in it than all his frescoes combined. The key to his ducal style is a comfortable emphasis on simple, solid forms and symmetry. It doesn’t call attention to itself. If, for political reasons, Vasari couldn’t evict the shops, he could at least put a lid on them. The picturesque jumble of red and yellow houses we find so charming must have pained the eyes of an image-conscious dictator like Cosimo I. Vasari laid the corridor like a roof on top of the ramshackle houses, containing them a bit. Then he punched a hole through the center with a classical arcade to give some dignity where it was sorely lacking. Vasari’s solution was ingenious, satisfying the duke’s craving for privacy and his refined aesthetics.

In time, the butchers were banished, and their places taken by the goldsmiths whose glittering windows make the Ponte Vecchio some of the most jealously guarded real estate in the world. Every year millions of tourists flock across the bridge, dazzled by the displays. Few look up to notice the corridor—today open only by appointment, its endless walls filled end to end with a treasure trove of Old Master paintings assembled largely by the Medici, a collection more priceless than all the gold on the street below. To the Romans, this bridge spelled empire, to the Grand Duke a secret passage disguised as a Renaissance face-lift, to the Allies a lifeline, to today’s visitor a shopping street of infinite charm—the Ponte Vecchio is many bridges, not just one.

Art critic John T. Spike lives at the foot of the Ponte alle Grazie, the Arno bridge to the east of the Ponte Vecchio.