An old wall town on the left bank, half way up the Cesano Valley. The town is built on a small hill slope while the borgo itself stands on the main road below it. This pleasant town bears the name of an abbey which was founded by Benedictine monks in the early Middle Ages and built using materials from the destroyed Roman town of Suasa Senonumwhich stood nearby. 
Passing under the entrance way leading up to the ancient castle we find the austere 16^th Century Palazzo Della Rovere, which now houses three museums - an archaeological museum, a museum of African ethnography and a natural history museum.
The attractive fortified villages of Montalfoglio (393 m) and San Vito sul Cesano (353 m) also form part of the district of San Lorenzo, each still surrounded with sturdy escarpment walls.