Purana Qila (Old Fort) is one of Delhi's oldest standing fortifications, built in the 1530s and 40s by both Humayun and his Afghan rival Sher Shah Suri. The massive walls and three ornate gateways are impressive examples of the transitional Mughal-Afghan architectural style. Inside the walls, the Qila-i-Kuhna Mosque — built by Sher Shah in 1541 — is one of the most beautiful mosques in Delhi, a forerunner of the great Mughal mosques to come.
A small museum inside the fort displays artefacts found during excavations on the site, which has been continuously inhabited since at least the 3rd century BCE and may be related to the ancient Indraprastha of the Mahabharata. The moat around the fort has been converted into a boating lake and children's park. Sound and light shows are held in the evenings.