By the sixth century BCE, the position of the Brahmins who specialized in ritual activity became questionable. The warrior class or Kshatriyas surfaced as a class of landowners. They desired a settled life based on agriculture and thus the introduction of the iron technology proved a boon for augmentation of agricultural surplus and clearing of forests. The middle Gangetic valley became the focus of increasing use of iron tools and wet rice cultivation. Larger food production made it possible to sustain increased production which is reflected in an increase in the number of settlements in the archeological records of the period from the sixth century to the fourth century BCE. The groups that grew up controlling surplus wealth became the ruling class of the newly emergent kingdoms. And on the foundation of this wealth were born the cities of the sixth century BCE.
The rise of cities in the sixth century BCE is mentioned in the Brahmanical, Buddhist and Jain texts of the times. It was this period which saw the beginning of the written tradition in ancient Indian literary history. This evidence of the emergence of cities is corroborated by the archaeological sources. In the upper Gangetic valley, people used a particular kind of pottery called the painted grey ware, whereas in middle Gangetic plains, black and red pottery was known. By about the sixth century BCE people of this entire zone started using Northern Black Polished Ware which is representation of the broad cultural uniformity in the Gangetic towns in the sixth century BCE. Punch marked coins made of silver and copper, probably issued by merchants, reflect organised commerce by this time. The introduction of money in turn led to the emergence of the class of money-lenders. Increased trade and developing economies led to massive fortification of the cities like Kaushambi, Ujjain, Rajghat(VaranWasi), Rajgir etc. These cities emerged as the centres of power and control over the Mahajanapadas. The use of terms ‘Pura and Durga’ to denote fortifications to protect urban centres and separate them for rural areas is an important indication for the rise of cities not only as seats of political power but as centres of commercial activity. The use of term ‘Nigama’ in Pali literature meant a township of specialized craftsmen. The term Nagara was commonly used for towns or cities which combined the political functions of the Pura and commercial functions of the Nigama. The Buddhist literature refers to six Mahanagaras located in the middle Gangetic valley namely Champa, Rajgriha, Kashi, Sravasti, Saketa, and Kaushambi.