The single-domed Qila-i-Kuna Mosque, built by Sher Shah in 1541 is an excellent example of a pre-Mughal design, and an early example of the extensive use of the pointed arch in the region as seen in its five doorways with the 'true' horseshoe-shaped arches. It was designed as a Jami Mosque, or Friday mosque for the Sultan and his courtiers. The prayer hall inside, the single-aisled mosque, measures 51.20m by 14.90m and has five elegant arched prayer niches or mihrabs set in its western wall. Marble in shades of red, white and slate is used for the calligraphic inscriptions on the central iwan, marks a transition from Lodhi to Mughal architecture. At one time, the courtyard had a shallow tank, with a fountain.
A second storey, accessed through staircases from the prayer hall, with a narrow passage running along the rectangular hall, provided space for female courtiers to pray, while the arched doorway on the left wall, framed by ornate jharokas, was reserved for members of the royal family. On a marble slab within mosque an inscription thus read, "As long as there are people on the earth, may this edifice be frequented and people be happy and cheerful in it". Today it is the best preserved building the Purana Qila.