Quwwat-ul-Islam mosque (Strength of Islam), also known as the Qutub Mosque or the Great Mosque of Delhi, to the north-east of minar was built by Qutbu'd-Din Aibak in AD 1198. It is the earliest extant mosque built by the Delhi Sultans.
It consists of a rectangular courtyard enclosed by cloisters, erected with the carved columns and architectural members of 27 Hindu and Jain temples which were demolished by Qutbu'd-Din Aibak as recorded in his inscription on the main eastern entrance.
The mosque is built on a raised and paved courtyard, measuring 141 ft (43 m). X 105 ft (32 m), surrounded by pillared cloisters added by Iltutmish between 1210 and 1220 AD. The stone screen between prayer hall and the courtyard, stood 16 mt at its highest was added in 1196 AD, the corbelled arches had Arabic inscriptions and motifs. Entrances to the courtyard, also uses ornate mandap dome from temples, whose pillars are used extensively throughout the edifice, and in the sanctuary beyond the tall arched screens.
What survives today of the sanctuary on the western side are the arched screens in between, which once led to a series of aisles with low-domed ceilings for worshippers. Expansion of the mosque continued after the death of Qutb.
Qutbuddin's successor Iltutmish, extended the original prayer hall screen by three more arches. By the time of Iltutmish, the Mamluk empire had stabilised enough that the Sultan could replace most of his conscripted Hindu masons with Muslims. This explains why the arches added under Iltutmish are stylistically more Islamic than the ones erected under Qutb's rule, also because the material used wasn't from demolished temples.
According to a Persian inscription still on the inner eastern gateway, the mosque was built by the parts taken by destruction of twenty-seven Hindu and Jain temples built previously during Tomars and Prithvi Raj Chauhan, and leaving certain parts of the temple outside the mosque proper.
Historical records compiled by Muslim historian Maulana Hakim Saiyid Abdul Hai attest to the iconoclasm of Qutb-ud-din Aybak. This pattern of iconoclasm was common during his reign, although an argument goes that such iconoclasm was motivated more by politics than by religion.
Close to the mosque is one of Delhi's most curious antiques, the Iron Pillar.
The 7 m-high iron pillar stands in the courtyard of the mosque. It is said that if you can encircle it with your hands while standing with your back to it your wish will be fulfilled.
The pillar carries a number of inscriptions and graffiti of different dates which have not been studied systematically despite the pillar's prominent location and easy access. The oldest inscription on the pillar is in Sanskrit, written in Gupta-period Brahmi script. This states that the pillar was erected as a standard in honour of Vishnu.
A deep socket on the top of the ornate capital indicates that probably an image of Garuda was fixed into it