The Great Mosque of Al Qala'a(Arabic: مسجد القلعة, romanized: Masjid El Qala'a) is a major historical Mosque located in the UNESCO World Heritage city of Qal'at Bani Hammad, in the province of M'sila, Algeria. Its construction dates back to the early 11th century, during the rule of the Hammadid Emir Hammad ibn Buluggin.

The Mosque spans an area of over 3500 square meters (38,000 sq ft), taking on a rectangular layout. It encompasses a vast courtyard, a hypostyle prayer hall, and a square minaret towering at 25 meters (82 ft) in height. Regarded as one of the largest historic mosques in Algeria, second only to the Mansourah, it also contains one of the country's oldest minarets. Additionally, the mosque likely exerted an influence on subsequent Almohad minarets, notably the Giralda in Seville. Together with other archaeological remnants of palatial structures, it stands as a primary testament to the opulence and impact of the Hammadid civilization.

The mosque was founded by the first Hammadid emir, Hammad ibn Buluggin, in the newly fortified city of Qal'at Bani Hammad, which would serve as the capital for the Hammadid Emirate. He hired skilled builders whom he brought from all corners of his domains. The Mosque is located in the lower part of the city, specifically in its southern section. According to Ibn Khaldun, it is assumed that the Great Mosque of El Qal'a was built in the year 398 AH, which corresponds to the year 1007 and coincides with the founding of the city, as is customary in the establishment of Islamic cities. Thus, the mosque was the first structure to be built, followed by the administrative center, and then the residential units.

The mosque was modified during the reign of Emir Al Nacir (1062–1088), who undertook a second phase of construction. It underwent transformations where the portico was expanded and extended to encircle the courtyard. The facade of the prayer hall was reinforced by closing some openings at the corners of the courtyard. Additional structures were added, and it was certainly during this phase that the maqsurah was built, with walls surpassing the roofs of the prayer hall. The entrances on either side of the minaret were closed off, and an additional floor was added to the corner constructions.

After the decline of the Qala'a of Beni Hammad due to the settlement of the Hilalian tribe in the region, the mosque fell into abandonment and then into ruins until the colonial period when General de Beylié led a three-and-a-half-month excavation campaign in 1908, partially uncovering the Palace of the Lake and the Mosque. Between 1964 and 1972, Dr.Rachid Bourouiba conducted research at the Qalaa of Beni Hammad, during which he unearthed the Great Mosque. In 1974, the restoration of the mosque's minaret took place; from 1976 to 1982, a UNESCO plan for the preservation and restoration of the site was implemented, and in 1987, an Algerian-Polish restoration mission was conducted on the Qal'a.

The mosque of the Qalaa, like all cities in the Islamic world, occupies a central place in relation to the city (the densely populated lower part). On one hand, on the North side, there are the palaces (palace of the lake or of the emirs, of the stars, and of salvation), and on the other hand, on the South side, there are the houses and other activities, and in front passes the main road serving the city and leading from Bab Djennane in the West to Bab al Akwass in the east. Over time, most of the mosque has Disappeared, leaving behind only remnants such as the minaret, foundation of the walls, and some columns. The mosque, originally of rectangular shape, stood at 63 meters tall and 53 meters wide, encompassed by a fortified wall bolstered by rectangular pillars. Within the mosque, a prayer room measuring 53 meters in height and 34 meters in width boasts thirteen arches and eight pavements. The courtyard, spanning 53 meters in length and 26 meters in width, is separated from the prayer house by a walled enclosure featuring three accessible doors in the surrounding wall.

The structure is surrounded by a 1.50-meter-high wall fortified with rectangular pillars on the outside, each measuring 1.80 meters in length and 1.30 meters in width, it features three open doors: one to the east, one to the west, and the third to the north. The northern door leads to a staircase of nine steps, currently blocked. Positioned on the side, the entrance leads into a courtyard with a portico and a cistern. The portico's cylindrical columns, crafted from white marble, once rested on masonry bases but are now incomplete, with all five columns on the east side missing. These main doors served as connections to the exterior, granting entry to a series of rooms or chambers covering the plateau, likely serving a purpose similar to the Mosque of Sultan al-Kalaun in Cairo.

The courtyard, spanning 53.20 by 26.90 meters, is encircled by a portico and paved with white tiles, with a cistern at its center supplied by a spring. This cistern, measuring 11.15 meters in length, 5.40 meters in width, and 2.80 meters in height, features a partially collapsed vault. Eleven doors in the surrounding wall lead into the courtyard from the prayer hall. To the west of the courtyard, three rooms dedicated to ablutions were found, one containing a large jar. Adjacent to the eastern section, a small room potentially serving as a library was uncovered. The eastern room, paved with stones and divided by a semicircular arch, stands on two uprights. The minaret is positioned centrally along the north side of the courtyard. According to Blanchet, the wall dividing the courtyard from the main hall featured vertically pierced pottery pipes for rainwater drainage. Additionally, within this wall was a semi-cylindrical niche, one meter wide, functioning as a mihrab and adorned with green faience. Fragments of stained glass windows were also discovered within the wall. Lastly, archaeologists found thirty-three sections of roughly hewn stone shafts in the courtyard, ranging in diameter from 15 to 42 cm.

At the heart of the mihrab axis, positioned in the center of the northern side of the courtyard, stands the minaret, currently reaching a height of 24.70 meters, albeit with its upper section collapsed. Its rectangular tower, measuring 6.50 meters on each side, houses a staircase with one hundred and twenty-seven steps, winding around its center and leading to the top. Constructed on a square plan reminiscent of those in Syria, it originally featured ceramic embellishments on its southern façade, while its wooden staircase was later replaced by one made of masonry.