Sher Shah built the Purana Qila fort in Delhi, which was later completed by Humayun. It features impressive architecture from its time including tombs, gateways, and a beautiful mosque. The fort has three arched gateways and was surrounded by thick ramparts and a moat connected to the Yamuna River. Within is the Qila-i-Kuna Mosque built by Sher Shah, which demonstrates a transition from Lodhi to Mughal architecture. The Sher Mandal tower was intended to be Humayun's personal library and observatory but he fell to his death there in 1556. Purana Qila contains remnants of the ancient city of Indraprastha but the dating of this is