Old Fort was built in the 16th-century by the Lion King, Sher Shah Suri. In 1539-40. The Old Fort, also known as the Purana Qil'ah or Purana Killa in Hindi and Urdu, is one of the most famous monuments in Delhi.

Delhi and Agra, bringing the Mughal Empire in India to a near end. Shah Suri constructed this enormous fort to defend his most prized possession, Delhi, from the Mughals. Though the fort did not see any major battle event, its very existence kept the Mughals away from invading Delhi until 1545 when Sher Shah Suri died. With the help of Persians, Humayun was able to re-capture Delhi and Agra.

Humayun made good use of the octagonal red sandstone tower known as Sher Mandal as his library and observatory. Some Mughal historical documents indicate that Humayun died as a result of the injuries he suffered after falling down from the steps of the Sher Mandal. Rumours exist that after the death of the Humayun, the Mughals, considering the fort to be cursed, vacated the structure. As a consequence, the Mughals had to construct a new fort, known as Lal Qi'lah, to defend Delhi from foreign invasions.

Recent archaeological excavations have exposed evidence such as pottery and other pieces of art and handicrafts which throws a new light on the existence of the city of Indraprastha, mentioned in the great epic of Mahabharata. A report by the Archaeological Survey of India suggests that the historic city of Indraprastha stood on the site where the Old Fort stands today. These findings have attracted the attention of a lot of archaeologists and Indian historians.

Today, the Old Fort houses the Delhi Zoo, Delhi's largest zoological park, and a boat club. The historic structure of the Purana Qila has stood witness to the restoration of the city of Delhi, the periods of anarchy, and the rise and fall of empires.