The Purana Qila, or old fort, is located on a site which has a long history of habitation. Excavations have reveled definite cultural layers continuously from the fourth/third centuries BC to the Mughal Period, and the pottery fragments associated with the lowest levels and could date from approximately 1000-500 BC. Popular tradition in fact links the site to the ancient city of Indraprastha which, accordking to the epic Mahabharata, was the capital city founded by the Pandavas on the bank of the river Yamuna. Though, the epic itself talks of the subsequent abandonment of the city, the memory of the place name lived on in the later village of 'Indarpat', that was within these walls till the begining of the twentieth century.
in 1533, the second Mughal emperor Humayun (ruled 1530-40 and 1555-56) decided to build a city that was called Dinpanah (the refuge of the faith) and he selected this site for the citadel of the city. It is belived that the walls and gates of the fort were completed within a year, and probably many other buildings were added in the next few years. Humayun's reign was interrupted in 1540 when Sher Shah Sur, the Afghan-origin chieftain from eastern India ousted Huamyun from the throne to capture Delhi and the Indian empire. More buildings were added to the fort by Sher Shah and his successors. In 1555 Humanyun regained the throne but then died within a few month.
The walls of the fort are 20 metres high and are 4 metres thick. They from a rough oblong with a perimeter of about 1.9km. There bastions at each corner of the fort and the walls, of which the western wall has 5 bastions. The eastern side was originally bounded by the Yamuna. but the reiver has long since changed its course eastwards. Aparts from the imposing gateways, very few buildings still remain within the fort. An ambiguous historical record makes it difficult for us to say for sure which of the structures of the fort were commissioned by Humanyun and which were commissioned by Sher Shah and his successor Islam Shah.