Djenné was founded between 800 and 1250 C.E., and it flourished as a great center of commerce, learning, and Islam, which had been practiced from the beginning of the 13th century. Soon thereafter, the Great Mosque became one of the most important buildings in town primarily because it became a political symbol for local residents and for colonial powers like the French who took control of Mali in 1892. Over the centuries, the Great Mosque has become the epicenter of the religious and cultural life of Mali, and the community of Djenné. It is also the site of a unique annual festival called the Crepissage de la Grand Mosquée (Plastering of the Great Mosque).
The Great Mosque that we see today is its third reconstruction, completed in 1907. According to legend, the original Great Mosque was probably erected in the 13th century, when King Koi Konboro—Djenné’s twenty-sixth ruler and its first Muslim sultan (king)—decided to use local materials and traditional design techniques to build a place of Muslim worship in town.
It is rectilinear in plan and is partly enclosed by an exterior wall. An earthen roof covers the building, which is supported by monumental pillars. The roof has several holes covered by terra-cotta lids, which provide its interior spaces with fresh air even during the hottest days. The façade of the Great Mosque includes three minarets and a series of engaged columns that together create a rhythmic effect.
At the top of the pillars are conical extensions with ostrich eggs placed at the very top—a symbol of fertility and purity in the Malian region. Timber beams throughout the exterior are both decorative and structural. These elements also function as scaffolding for the re-plastering of the mosque during the annual festival of the Crepissage.
According to khanacademy.org