Purana Qila is the fort that Humayun began around 1533 as Dinpanah and that Sher Shah Suri completed after driving him out, adding walls roughly 1.5 km in circuit with three great gates — Bara Darwaza to the west, Humayun Darwaza to the south and Talaqi Darwaza to the north. Inside stand two exceptional buildings: the Qila-i-Kuhna Masjid of 1541, a five-arched mosque in red sandstone and marble, and Sher Mandal, an octagonal two-storey pavilion that Humayun used as a library and from whose steps he fell to his death in 1556. What makes the site archaeologically important is what lies beneath: ASI excavations, resumed in recent seasons, have exposed Painted Grey Ware, Northern Black Polished Ware and later levels in a sequence long argued to represent the site of Indraprastha. A visit is short and physical rather than gallery-like. You come through the Bara Darwaza onto a broad ramp, and the whole interior is essentially a grassy plateau ringed by ramparts. The Qila-i-Kuhna Masjid sits to your right, its mihrab wall carved with marble inlay and a stair to the roof; the sandstone is warm and the interior acoustics are startling. Sher Mandal stands a hundred metres south, small, red and steep, with the fatal staircase roped off. Between the two, fenced excavation trenches show the stratigraphy, with signboards giving the ceramic phases. Walk the inner face of the ramparts to the Talaqi Darwaza for the best wall views, and look out at the lake on the west side, dug in the 1960s along the old moat line, where pedal boats run. Ninety minutes is honest, two hours if you walk it all.