Delhi, officially known as the National Capital Territory of Delhi (NCT), is the second most populated metropolis in India after Mumbai, with a population of 17 million. The city is also the eighth most populated metropolis in the world. Delhi is known to have been continuously inhabited since the 6th century BC. Through most of its history, Delhi has served as a capital of kingdoms and empires, the fact of which is very well proved by the erection of many historical sites and monuments across the city. Delhi has been invaded, ransacked and rebuilt a lot of times, particularly during the medieval era, and therefore the modern city of Delhi is a cluster of many cities scattered across the metropolitan region. Delhi now has re-emerged as a major political, cultural and commercial city along the trade routes between northwest India and the Gangetic plain after the rise of the Delhi Sultanates.