The visible external structure of the Fatehpur Sikri step-well is an octagon. Thirteen steep steps ascend from either end of the north and west faces to the flat stone-paved roof, where an octagonal platform projects both on the northern and southern sides, marking the roof of the room in which revolved the great windlasses for raising the water. The flights of steps leading down to the water are so contrived that they never meet nor interfere with the windlasses in the upper southern and northern chambers. This well was built primarily to supply the haram sara and some other parts of the palace with water by mechanical means. It is built of red sandstone, and its corridors are decorated with rosettes and simple carvings.
Sources:
Asher, Catherine. The New Cambridge History of India: Architecture of Mughal India, 21-22. Cambridge: Cambridge University Press: Cambridge, 1992.
Rizvi, Saiyid Athar Abbas. Fathpur-Sikri. Bombay: D. B. Taraporevala Sons, 1975.
