The unimaginatively named 'Old Fort' is where Mughal Emperor Humayun met his end in 1556, tumbling down the steps of the Sher Mandal, which he used as a library. The fort had been built by Afghan ruler Sher Shah (1538–45), during his brief ascendancy over Humayun. It's well worth a visit, with its peaceful garden studded with well-preserved ancient red-stone monuments, including the intricately patterned Qila-i-Kuhran Mosque (Mosque of Sher Shah).
A popular boating lake has been created from Purana Qila's former moat, with pedalos for hire.
Across busy Mathura Road are more relics from the city of Shergarh, including the beautiful Khairul Manazil mosque, still used by local Muslims and favoured haunt of flocks of pigeons.