The southern gateway of the Quwwatu`l-Islam mosque, as extended by Ala-ud-Din Khalji, is known as the Ala-i-Darwaza and several inscriptions executed to form an ornamental surface, three mention the date of its erection as 710 A.H. (1311). Alai Darwaza of Delhi is the first building employing completely the Islamic principles of accurate construction and geometric ornamentation and also shows Saljuqian characteristics, which influenced the Khalji architecture. The characteristics of Alai Darwaza , Delhi include wide and bulging dome with a central knob, pointed horse-shoe-shaped arches and squinches and lotus-bud fringes of the arches. The celebrated gateway, is built of red sandstone, and is 17.2m squares with arched openings on all sides, and is surmounted by a wide but shallow dome on an octagonal base achieved through squinches with concentric series of arches. The northern arch is semicircular, while others have a pointed horseshoe shape, with radiating voussoirs laid on the principle of true arch.
The underside of the arches is fringed with lotus-bud embellishment, not merely in the openings, but also in the perforated side-windows. Excellent proportions, profuse geometrical carvings on the interior, inscriptional bands of white marble in `Naskh` characters and other decorative details in red stone make it a very pleasing structure. It has been described as "one of the most treasured gems of Islamic architecture".