Without the use of cement the Romans would not have been able to successfully build the barrel vault that supports the structure's ramp system. The sanctuary is built into the hillside so that the structure is embedded in the hill. The building contains seven terraces, 400 feet high, from where diviners helped pilgrims reach the colonnaded rotunda at the top containing the 17th-century palace of the Barberini. The structure is intended to overlook the plain and the mountains, and has an axial plan