Timbuktu is a historic city located north of the Niger River in the Tombouctou Region of Mali. The city developed as an important trading center for the trade in salt, gold, ivory and slaves in the 11th and 12th centuries AD, and then it reached its zenith during the era of the Mali Empire in the 13th and 14th centuries, and also after that during the era of the Songhai Empire. Timbuktu was once home to the Quranic University of Sankore (Sankore Madrasah), founded within the Sankore Mosque. In the 15th and 16th centuries, the city became an intellectual and spiritual capital and a center for the spread of Islam throughout Africa with its three great mosques, Djinguereber, Sankore and Sidi Yahia. The historic city of Timbuktu was included in the UNESCO World Heritage List in 1988, as part of Timbuktu.