The Sher Mandal named so after Farid (sher shah) who had tried to finish what was orderd by Babur but had died during the initial phase and so construction was halted until the arrival of Humayun.
This double-storeyed octagonal tower of red sandstone with steep stairs leading up to the roof was intended to be higher than its existing height. Its original builder was Babur who orderd the construction and was used as a personal observatory and library for his son Humayun, finished only after he recaptured the fort. It is also one of the first observatories of Delhi, the first being in Pir Gharib at Hindu Rao at Ridge built in 14th century by Firoz shah Tughlaq. The tower is topped by an octagonal chhatri supported by eight pillars and decorated with white marble in typical Mughal style.
Inside there are remnants of the decorative plaster-work and traces of stone-shelving where, presumably, where the emperor's books were placed.
This was also the tragic spot where, on 24 January 1556 Humayun fell from the second floor to his death. He slipped while hasting to the evening prayers, following his hobby of astronomical star gazing at the top of this private observatory. He fell headlong down the stairs and died of his injuries two days later.However, entry inside the library is now prohibited
Read more about this topic: Purana Qila, Delhi