San Marco is a typical hill castle. Its current aspect reflects its re-building in the 13th and 14th centuries although it is older in origin.

In the early Middle Ages it must have stood higher on the hills, perhaps near the church of San Marco Vecchio. It was a fortified, rural village guarding the paths on the plain of S. Scolastica and the route to the Via Salaria.

The buildings appear compact in good condition, only partly rebuilt and all contained within the town walls still intact with angular, cylindrical towers. The buildings are interesting and repeat the plan of terraced homes and haylofts along a central axle with orthogonal side-roads. Oral tradition says that the place was an outpost for the Republic of Venice. It was probably an ingenuous attempt to give the settlement and therefore its inhabitants a historic background that explained the name of the town and the uncommon beauty of its fortifications. The inhabitants say that the oldest settlement lies near the church and cemetery.