Purana Qila ( Old Fort ) is one of the oldest forts in Delhi, India.
Its current form was built by the Afghan king Sher Shah Suri,
on the site which was perhaps that of Indraprastha, the legendry
capital of the Pandavas. Sher Shah raised the citadel of Purana Qila
with an extensive city-area sprawling around it.
It seems that the fort  was still incomplete at Sher Shah' s death in 1545,
and was perhaps completed by his son Islam Shah, although it is not
certain which parts were built by whom.
See Wikipedia for further details.