Throughout India's history the region of Delhi, commanding roads in all directions, was the key to empire. From the earliest times many cities rose and fell there, and within 50 sq mi (130 sq km) S of New Delhi are more important dynastic remains than exist in any other area of the country. The earliest city on the Delhi plain was the semilegendary Indraprastha, mentioned in the Hindu epic Mahabharata. Another historic site is the Rajput citadel and town containing the Lal Kot [red fort], erected in 1052; it is sometimes confused with Shah Jahan's Red Fort in Old Delhi.

In 1192 the legions of the Afghan warrior Muhammad of Ghor captured the Rajput town, and the Delhi Sultanate was established (1206). The invasion of Delhi by Timur in 1398 put an end to the sultanate; the Lodis, last of the Delhi sultans, gave way to Babur, who, after the battle of Panipat in 1526, founded the Mughal empire. The early Mughal emperors favored Agra as their capital, and Delhi became their permanent seat only after Shah Jahan built (1638) the walls of Old Delhi. Among the most famous monuments on the Delhi plain are the 12th-century Kutb Minar and the tomb of Humayan (built 1565–69; it is the architectural prototype of the Taj Mahal at Agra).

The city of Delhi, or Old Delhi (1991 pop. 7,175,000), on the Yamuna River, adjoins New Delhi in the east central part of the state and is a commercial center. It was enclosed by high stone walls erected in 1638 by Shah Jahan. Within the walls he built the famous Red Fort—so called for its walls and gateways of red sandstone—that contained the imperial Mughal palace. The fort remained a military garrison until 2003. In the palace is a public audience hall (Diwan-i-Am), where the splendid Peacock Throne stood, and a private audience hall (Diwan-i-Khas), built entirely of white marble and bearing the apt inscription If there is a heaven on earth, it is this! Shah Jahan also built the Jama Masjid [great mosque], one of the finest in Islam. Just south of the fort, on the Yamuna's bank, is Rajghat, where the bodies of Mohandas Gandhi and of India's prime ministers have been cremated; it is now one of the most revered shrines in India. In the northwest, beyond the old walls, is the Univ. of Delhi.

The present city of Old Delhi did not become important until Shah Jahan (for whom it was sometimes called Shahjahanabad) made it the capital of the Mughal empire in 1638. It was sacked (1739) by the Persian Nadir Shah, who carried off the Peacock Throne. The city was held by the Marathas from 1771 until 1803, when the British took it. During the Indian Mutiny of 1857 it was held for five months by the rebel soldiers. Delhi Cantonment was (1912–31) interim capital of India until New Delhi was officially inaugurated.