Before the Black Death devastated Siena, the city thrummed with energy, expressed in art and architecture designed to dazzle its audience – and which still astonishes 800 years later

If you want to know the moment of a medieval Italian city’s greatest prosperity, look at the year it began work on its cathedral. In Siena, the magic year was 1226, the start of some 85 years of construction of the duomo, a remarkable gothic structure with an intricately complex, creamy pink facade and stripy, black-and-white campanile. “The scale of ambition is difficult to put into words,” says Laura Llewellyn, one of the curators of The Rise of Painting, the National Gallery’s new exhibition of Sienese art. “The extravagance of it: to appreciate it you need to unknow and unlearn later buildings like the duomos in Florence and St Peter’s in Rome.”

But by the 1350s, Siena’s most glorious years in the raging Tuscan sun would be as good as over. After decades of rapid artistic transformation – a half century that saw the art of the city leave behind the distant, hieratic grace of Byzantine-flavoured painting for a world of dynamism, drama and emotion – the Black Death halved the city’s population from 60,000 to 30,000, stripped away its wilder ambitions and dulled its gleaming wealth. One of Siena’s more implausible plans had been to enlarge the already huge cathedral by converting its existing nave into a transept and tacking on to its belly a new, vastly oversized nave on the precipitous edge of one of the city’s peaks. The project was never completed, but ghostly unfinished arches remain as a monument to lost dreams and a raging pandemic.