"When justice is sovereign, the wicked are stripped of all power"

SIENA IS ONE OF THE GREAT HISTORIC CITIES – a living vision of the medieval world found now in very few places. Here is the architecture and street plan of a medieval city-state, the oldest bank in the world, and one of the oldest universities. Siena was even the first place to ban the car, in 1966! In its centre, the great square called the Campo hosts the famous Palio horse race twice a year. It's a place to relish the continuance of the past into the present.

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In the Middle Ages, Siena was run by the Nine, who rotated every two months as a bulwark against tyranny. They met in the council hall in the Palazzo Pubblico (Public Palace) on the Campo. On the first floor is a room adorned with some of the most extraordinary paintings in western art. Three fresco panels, painted by Ambrogio Lorenzetti in 1338, offer a vibrant vision of life in town and country on the eve of the Black Death. This huge work, known as The Allegory of Good and Bad Government, covers three walls. In the dim light, the eyes need a moment to take in the scale and the colour – the deep, luminous ultramarine skies created with a paint made from ground lapis lazuli. It’s the last great creation of the school of painting in Siena that came to an abrupt end when the plague struck Italy in 1348, killing half the city’s population - including Lorenzetti and his elder brother, Pietro.

These extraordinary frescoes came to mind while visiting the National Gallery’s fabulous Siena exhibition this summer. The focus there was on four painters: Duccio, Simone Martini and the Lorenzettis. It encapsulated an exquisite, idiosyncratic moment, with none of the advances of Renaissance art: perspective, anatomy, realism.