The Sidi al-Kettani Mosque, also known as Salah Bey Mosque, is one of “the main mosques of Constantine. The Mosque is an impressive stone structure, rectangular in plan, roofed with traditional terracotta tiles. A cylindrical minaret with a typical “pencil point” cap rises in the north- east corner. The mosque presents three facades to the street. The fourth wall abuts on the adjoining Sidi al-Kettani madrasa to the north. The west façade is articulated in a series of blind arches at ground floor level and semi-circular arched windows at upper floor level.

The other two facades are more elaborately decorated, each comprising a central entrance door flanked by shops at ground level and by semi-circular arched windows at the upper level. The south façade has its ornamentation concentrated on the central doorway, which seems to have been the main entrance to the mosque in the time of Salah Bey as it is adorned with an inscription carved on a marble panel giving the date of its construction 1776.

The original entrance in the south façade gives direct access to an open courtyard surrounded by an arched portico and containing in its centre a fountain. A doorway in the east wall of the courtyard opens into a corridor leading to the present entrance porch (in the east façade) past rooms where Koran
instruction is given to children. A remarkable semi-circular mihrab-like alcove containing a bench seat in the north wall of the courtyard is flanked by two flights of marble stairs leading up to a prayer hall on the upper floor. A room for ritual ablutions is located in a low barrel-vaulted chamber
under the stairs.

The prayer hall of the mosque is unusual in being in the upper floor. Five carved wooden doors lead from a gallery around the courtyard to the prayer hall, which has five arcades running parallel to the qibla wall intersected by a central nave leading to the mihrab. The arcades are supported by white marble columns and the central nave is roofed by three eight faceted domes resting on octagonal drums on fluted squinches. A tiled dado runs around all four sides of the prayer hall. The mihrab in the centre of the qibla wall is elaborately decorated with carved plasterwork. A timber gallery facing the qibla wall provides accommodation for women. It is reached by two wrought-iron spiral staircases at either end and is fitted with a small projecting dikka (bench) from where the words of the imam could be repeated by the muballigh to ensure that the women might hear the service clearly. To the right of the mihrab stands a richly sculptured marble minbar.

The Salah Bey Mosque is distinguished by the variety of its capitals. The influence of ancient architecture is noticeable in the cruciform springs above the prismatic columns and the acanthus leaf capitals inspired by Roman composite capitals.

The Anatolian type plan and its proportions make this mosque an extremely original building of the Ottoman period. A courtyard precedes the mosque; an exterior portico added in 1852 precedes a main building, one of the rooms of which served as an audience hall and the other as a Koranic school. The prayer room is accessed by an entrance located in the axis of the mihrab which opens onto a gallery delimiting on three sides a space in front of the mihrab; divided into nine bays, the gallery is covered with domes: three identical side domes face each other, while to the west, the smaller bays delimit slightly elliptical domes. In the centre, the space is covered with a horseshoe dome on pendentives, pierced by eight windows in a raised arch and a cornice; the whole was covered with polychrome drawings during the colonial period. This very rare plan for a mosque is found in Algiers in the Kafar mosque. It is the second mosque with a hemispherical dome with that of Ketchawa, in Algeria. The mosque is also one of the few mosques with several side galleries and at the back of the courtyard without pillars. In the prayer hall, both the pillars and columns alternate as in the mosques of Raqqa, Tinmal, the Qarawiyyîn of Fez.