Every city in Tunisia has a Great Mosque, but the Great Mosque in Kairouan is the most important of all. It is the oldest Muslim place of worship in Africa and is commonly regarded as the fourth holiest site in Islam (after Mecca, Medina, and Jerusalem).
History
The first mosque was built in Kairouan in 670, shortly after the arrival of Muslim Arabs to North Africa. It was built by the founder of Kairouan, Sidi Oqba (the mosque is also known as the Sidi Oqba Mosque). The current mosque was built by Emir Abou Ibrahim Ahmad in 863.
For centuries, the Great Mosque of Kairouan has served as a pilgrimage destination for North Africans not able to make the long trip to Mecca. According to popular belief, seven trips to Kairouan was worth one hajj to Mecca.
What to See
The exterior of the Great Mosque of Kairouan, with buttressed walls, is in the typically austere Aghlabid design, but the grand minaret indicates the buildings significance and the mosque becomes far more impressive upon entering.
Muslims can enter the mosque through nine different gates; non-Muslim visitors use the main gate on rue Oqba ibn Nafaa and must be appropriately dressed; robes are available at the entrance for those who are not.
The courtyard is paved with flagstones and slopes to intricately decorated draining hole in the center, which filters dust from the rainwater and deposits it into a 9th-century cistern. Around the courtyard are beautiful horseshoe-arched porticoes containing some 400 ancient pillars. These were reused from Roman, Byzantine, and Latin Christian buildings, and symbols of these former faiths can be seen throughout.
On the north side of the courtyard is a massive, three-story minaret that rises 115 feet high. The lowest level of the minaret dates from 728 and includes two reused Roman slabs with Latin inscriptions (one upside down).
Having removed their shoes, Muslims perform the customary ablutions (ritual washing) in the portico before entering the prayer hall, which is topped by a fine dome and entered through beautifully-carved wooden doors dating from 1829.
Non-Muslims are not allowed inside, but the doors are left open to allow glimpses of the interior, which consists of 17 aisles supported by 414 ancient columns of marble and porphyry from Carthage and Sousse. The hall is illuminated by chandeliers and softened with rugs that cover the entire floor and the bases of the columns.
The central aisle leads to the 9th-century tiled mihrab (niche indicating the direction of Mecca) on the south side of the mosque. The tiles of the mihrab, as well as the wood for the nearby minbar (pulpit), were imported from Baghdad. Also inside the mosque are tombs of local saints.
Source: http://www.sacred-destinations.com/tunisia/kairouan-great-mosque