Old Fort

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, India and is its oldest historical site. The fort was constructed in the 16th-century by the founder of the Sur Dynasty, Sher Shah Suri. The fort's unique Mughal-Hindu-Afghan architecture makes it a popular tourist destination in Delhi. However, in recent years, the fort has also attracted a lot of attention of archaeologists. Recent archaeological evidence suggests that the historic city of Indraprastha once stood where the Old Fort is today.

More info at  en.wikipedia.org