Purana Qila (Old Fort) stands on a site believed to have been continuously inhabited since at least 1000 BC -- tradition identifies it with Indraprastha, the legendary capital of the Pandavas from the Mahabharata. The current walls and gates were built by Mughal emperor Humayun in the 1530s and completed by Sher Shah Suri after he seized power.
The massive rubble-stone walls stretch over a kilometre and rise to 18 metres in places. Inside, two buildings stand out: the Qila-i-Kuhna Mosque, an elegant single-dome mosque with fine stone carvings, and the Sher Mandal, an octagonal tower that Humayun used as a library and where he fatally fell down the stairs in 1556.
The fort is surrounded by a lake where you can rent pedal boats, and the grounds are surprisingly peaceful given that this is central Delhi. A sound-and-light show runs in the evenings. Entry is modest and the site rarely feels crowded.