Purana Qila (Old Fort) is one of the oldest forts in Delhi. Its current form was built by Sher Shah Suri, the founder of the Sur Empire. Sher Shah raised the citadel of Purana Qila with an extensive city-area sprawling around it. It is believed that the Purana Qila was still incomplete at Sher Shah's death in 1545, and was perhaps completed by his son Islam Shah, although it is not certain which parts were built by whom.
The fort was the inner citadel of the city of Din Panah during Humayun's rule who renovated it in 1533 and completed five years later. The founder of the Suri Dynasty, Sher Shah Suri, defeated Humayun in 1540, naming the fort Shergarh; he added several more structures in the complex during his five-year reign. Purana Qila and its environs flourished as the "sixth city of Delhi".
When Edwin Lutyens designed the new capital of British India, New Delhi, in the 1920s, he aligned the central vista, now Rajpath, with Purana Qila. During the Partition of India, in August 1947 the Purana Qila along with the neighbouring Humayun's Tomb, became the site for refuge camps for Muslims migrating to newly founded Pakistan. This included over 12,000 government employees who had opted for service in Pakistan, and between 150,000–200,000 Muslim refugees, who swarmed inside Purana Qila by September 1947, when Indian government took over the management of the two camps. The Purana Qila camp remained functional till early 1948, as the trains to Pakistan waited till October 1947 to start.
In the 1970s, the ramparts of Purana Qila were first used as a backdrop for theatre, when three productions of the National School of Drama were staged here: Tughlaq, Andha Yug and Sultan Razia, directed by Ebrahim Alkazi. In later decades it has been the venue of various important theatre productions, cultural events, and concerts. Today, it is the venue of a daily sound and light presentation after sunset, on the history of the "Seven Cities of Delhi", from Indraprastha through New Delhi.
The walls of the Fort rise to a height of 18 metres, traverse about 1.5 km, and have three arched gateways: the Bara Darwaza (Big Gate) facing west, which is still in use today; the south gate, also popularly known as the 'Humayun Gate' (probably so known because it was constructed by Humayun, or perhaps because Humayun's Tomb is visible from there); and lastly, the 'Talaqi Gate', often known as the "forbidden gate". All the gates are double-storeyed sandstone structures flanked by two huge semi-circular bastion towers, decorated with white and coloured-marble inlays and blue tiles. They are replete with detailing, including ornate overhanging balconies, or jharokhas, and are topped by pillared pavilions (chhatris), all features that are reminiscent of Rajasthani architecture as seen in the North and South Gates, and which were amply repeated in future Mughal architecture. Despite the grandeurs of the exterior, few of interior structures have survived except the Qila-i Kuhna Mosque and the Shermandal, both credited to Sher Shah.