The Benedictine abbey is planted on a ridge of grey shale like a square turret on a battleship. It is the seat of the Olivetans, a sect founded by a Sienese bishop in 1313. In its austere halls paintings, ceramics and marquetry show up to advantage. The abbey’s priceless treasure is the cycle of 36 frescos in the great cloister. They portray the life of St Benedict, who was born just across the regional border in Umbria and is now patron saint of Europe, and are the joint work of Luca Signorelli and Giovanni Sodoma. Some of the monks here are scientists, engaged in metallurgy.