Tughlakabad Fort is the ruined 14th-century fortress of Ghiyasuddin Tughlaq, built as the third city of Delhi. Its massive stone walls and broken gateways still give a strong sense of the scale of the medieval city.
The site is rougher than Delhi's better-kept monuments, but that is part of the appeal. Go for the fortress walls, the views across the ruins, and the nearby mausoleum of Ghiyasuddin Tughlaq.