Khirki Mosque, a fort-like pre-Mogul structure of Delhi

unique fort-like design, Khirki Mosque, Delhi. archnet.org

Fort like look, Khirki mosque, Delhi. tripadvisor in.n

Unlike countless masjids built here in Delhi or elsewhere the Khirki Masjid structurally looks different. The local people of Khirki Village in South Delhi call it ‘the qila’ (fort) rather than as ‘the masjid’. Kirki means window, hence it is called a ''Mosque of Windows''. It is close to the Satpula or the seven arched bridge on the edge of southern wall of Jahapanah (the fourth city of Medieval Delhi).

Khirki Mosque, Khirki, Delhi en.wikipedia.org

The very first glance at this structure won't impress you as a place of Muslim worship. The reason is its massive sloping rubble walls, its corner domed towers and rubble masonry front part , would make it look like a fortified building, certainly not a holy place. Khirki Masjid, with a large square plan, was built by Khan-i-Jahan Junan Shah, the Prime Minister of Feroz Shah Tughlaq (1351–1388) of the Tughlaq Dynasty (during Firoz Shah’s reign (probably in 1375 AD). Some historians suggest the structure is dated back to the 1350s The two-story structure has four domed towers at four corners and they appear to bulge out. The lower level has rows of shallow, arched cells and above them is the entrance where worshipers would enter the masjid to offer Namaz/ prayer.

Khirki Mosque, Khirki, Delhi en.wikipedia.org

Central arcade, Khirki Mosque, Delhi.en.wikipedia.org

The unusual aspect of this mosque is the intentional fusion of Islamic and traditional Hindu architecture and further, this structure seems to be the only monument-style mosque in North India, which is mostly covered. During the Muslim rule of North Karnataka, in places like Gulbarga such covered mosques were built in the past.
The construction date of this mosque is under debate. The inscriptions on the eastern gate of the Mosque say the builder was 'Khan-e-Jahan Junaan Shah'. Some historians in their study "The Tughluqs: Master Builders of the Delhi Sultanate" made comparison with other construction activities of that period. It is dated between 1351 and 1354 when Feroz Shah Tughlaq, during his stay in Jahapanah, ordered this mosque to be built as "his pious inaugural contribution to the Capital"

The main structure is raised on a plinth of 3 m (9.8 ft). There are four open courtyards surrounded by arcades built with 180 square structural columns and 60 pilasters. They run in north–south direction and divides into aisles. In the interior, bays of arcades signify non-hierarchical space. The advantage of open courtyards is it is a source of light and ventilation to the interior prayer areas. It is quite comfortable during Namaz in the summer time. As for the roof, it has 25 partitioned squares of equal size with 9 small domes in each square. It means a total of 81 domes with alternating 12 flat surface to cover the roof. The four corners of the mosque are adorned with domed towers that look like watch towers. . There are 3 protruding gateways at the corners of the mosque, fourth one is absent or missing? The turrets that flank the south and north gates are circular in shape; their articulation gives them a three storied appearance.
The main gate, which leads to the qibla on the western wall, has a projecting mihrab. There are arch windows (carved out of stone guard) with perforated screens or jalis or tracery on the vaulted first floor cells. They are known as "Khirkis", seen on the second floor.
The mosque's walls are made of typical rubble masonry construction with plastered surface on the outside. The interior walls are simple with traditional carved stone screens.

Khirki Mosque, roof terrace. flicker.com

The presence of large number of stone screen windows in this mosque is a slight departure from the earlier style and the Khirki Mosque's unique architectural element and style is widely interpreted as a precursor to the intensely ornamental-rich Mogul style of architecture (1526-1857) and of the Lodhi 's (1451–1526.
The salient features are: interiors are undercoated but for traditional carved stone screens (jalis) that also admit light and provide ventilation. The rubble core construction of the mosque's walls, both exterior and interior, can be seen in places where the plaster has fallen off., Fort-like look., nicely made stone-screen windows.
This mosque was in a state of neglect for a long time and this resulted in the collapse of a few domes and the dilapidated condition of some parts of the mosque. Delhi chapter of Indian National Trust for Art and Cultural Heritage (INTACH) categorized the monument as "Grade A" in terms of archaeological value and was a partner in the restoration work. Way back in 2010, the ASI recognized this mosque among the 43 monuments to be restored before 2010. The ASI was not happy with the restoration work done earlier and have a proposal to try new methods with use of lime in correct quantity.
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https://en.wikipedia.org/wiki/Khirki_Mosque