Delhi is India's capital and seat of government. It forms the National Capital Territory of Delhi, rather than being part of a state. Delhi is one of India's largest cities, and the core of one of the largest metropolitan areas in the world, with over 21 million inhabitants. Within India it is a major center of arts, commerce, education, tourism, and transit. As the capital of several empires over the last 2000 years, Delhi also contains a striking array of well-preserved historic sites for the tourist to visit. With evidence of continuous settlement dating back to the 6th Century BC, Delhi is one of the oldest inhabited cities in the world. Thought to have been built and destroyed eleven times, evidence of at least eight distinct settlements can still be seen in Delhi. The most well-preserved historic sites are from the period of Muslim rule, between 1193 and the 1700s.