 The largest ones were between the city of Mathura (on the Yamuna river) and Magadha (on the Ganges).Another series of forts in the south, was on the Ujjain (on the Narmada) leading into the Deccan.These are inferred by the remains of fort walls and bastions seen on excavation at Rajagriha and at several sites in the Gangetic plain notably Kaushambi.