Purana Qila (Old Fort) is one of Delhi's oldest fortresses, built by Sher Shah Suri in the 16th century. It stands on the ruins of ancient Indraprastha city. Features the Qila-i-Kuhna Mosque and Sher Mandal library where Emperor Humayun fell to his death. The fort has a moat for boating and hosts a spectacular light and sound show.