From Wikipedia: Purana Qila (Hindi: पुराना क़िला, Urdu: پُرانا قلعہ, translation: Old Fort), is the oldest fort among all forts in Delhi and, the oldest known structure of any type in Delhi. It was re built by the Afghan king Sher Shah Suri, on the same site, which was perhaps the site of Indraprastha, believed to be the capital of the Pandavas, Sher Shah raised the citadel of Purana-Qal’a with an extensive city-area sprawling around it. It seems that the Purana-Qal’a was still incomplete at Sher Shah’s death in 1545, and was perhaps completed by his son Islam Shah or Humayun, although it is not certain which parts were built by whom. It’s located at the site of the legendary city of Indraprastha, that was founded by Pandavas on the banks of perennial river Yamuna, which is revered by Hindus since ages, points to the possibility of this site’s history dating back to nearly more than 5000 years old. Consequently the fort is considered by some, to be ‘the first city of Delhi’. Researchers now confirm that uptill 1913, a village called Indrapat existed with in the fort walls. Excavations carried out by Archaeological Survey of India (ASI) at Purana Quila in 1954-55 (trial trenches) and again 1969 to 1973 by its Director, B B Lal have unearthed Painted Grey Ware dating 1000 B.C., and with a continuous cultural sequence from Mauryan to Mughal through Sunga, Kushana, Gupta, Rajput and Sultanate periods, confirming the antiquity of the fort.