Three major methods were used for the construction of ancient Indian forts. The first consisted of earthen ramparts. Often they were constructed of the sand which was dug out of the ditch surrounding the fort. The second of rubble with earth on the outside which was more sturdy. The third type of construction was with stone and masonry work. The last was the strongest. Often materials from demolished forts were reused in the building of new forts.[2] By 4 BCE, fortified cities were common in India. 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. At the latter site huge walls of burnt brick, which look like they have been battered. There does not seem to be any formal planning of these forts.[3]