Purana Qila, the 16th-century walled citadel, was built on the legendary site of Indraprastha, by rival Emperors Humayun and Sher Shah. Inside it stands Sher Shah’s most outstanding monument, the well-preserved Qal’ a-i-Kuhna mosque (1541), built in the typical Mughal style of sandstone, inlaid with marble. Its five great arches, decorated mihrabs, and marble-inlaid sandstone facade mark the change from Lodi to Mughal architecture.
This mosque is midway in style between the solid, simple mosques of the Sultanate period and the more graceful and sophisticated ones of the later Mughal period. Immediately, south stands a two-storied octagonal tower of red sandstone, used as a library and an observatory. This is where Humayun accidentally met his death in 1556.
This is one of the finest examples of Indo-Afghan architecture. The arches and rich ornamentation in black and white marble, against red sandstone, became very popular during the Mughal Period.