The arable land was contained within an enclosure of volcanic highlands and the Tiber River. The northern border of the enclosure went along the ridge of the Monti Cimini, the southern along the ridge connecting the Monti Sabatini and Monte Soratte, the western along the highlands connecting the two large volcanic lakes. The inner slopes are drained by streams pointing at the Tiber, which collect into converging canyons and finally into the canyon of the Treia River, which empties into the Tiber. These streams required an extensive network of bridges.