The Great Iwan, or Iwan al-Kabir, was located in the southern section of the Citadel on the site of earlier halls.  al-Nasir Muhammad rebuilt the monumental structure twice, in 1315 and 1334.
The plan documented in the 19th century for inclusion in the Description de l'Egypte shows  a square hypostyle structure with five parallel aisles and a dome.  The building was open to the exterior on three sides through arcades, and the main façade was articulated with  a large central arch flanked by two smaller arches on either side.
The Great Iwan was a public and ceremonial space where the Sultan sat enthroned to administer justice, receive ambassadors, and carry out other duties of state.
Source:
Rabbat, Nasser O. 1989. Citadel of  Cairo (Geneva: AKTC), p. 11-13.
Gillispie, Charles Coulston & Michel Dewachter. 1987. Monuments of Egypt : the Napoleonic edition : the complete archaeological plates from La description de l'Egypte.  (Princeton, NJ : Princeton AP & Architectural League of NY, The J. Paul Getty Trust).