Located 200 miles east of Timbuktu, Gao was one of the key terminals of trans-Saharan trade through Mali, which marketed such goods as copper, salt, Venetian beads, and Damascene and European sword blades, in exchange for slaves and gold. According to oral tradition, Sorko fishermen from the south, who had migrated up along the Niger River, first settled the area around Gao in the late seventh century. In the eleventh century the Songhay people settled 100 km south of where Gao is today, concurrent with the conversion of their king to Islam. Archeological evidence suggests that at this time the growing metropolitan area was split in two distinct districts, one for the royal residence and mosque, and the other the residences of the people and the market. It has been suggested that the site of the Tomb of Askia may have represented an intermediary and even sacred open space between the two, and may have been used by the people as a musalla for prayer. Twelfth and thirteenth-century tombstones inscribed with kufic script have been found in underground vaults in Gao marking what is believed to be the old residential and commercial districts of the city. 

By the early fourteenth century, the strength of the Songhay state had declined. In 1325 the Emperor of Mali, Kankan Musa, invaded Gao and wrested control of the large trading area to the north. Kankan Musa had a strong impact on the built environment of Gao. Under his rule, the first flat roofed houses in both Gao and Timbuktu were built. He also commissioned the first Friday mosque in Gao, which incorporated a circular kiln-dried brick mihrab. Though this mosque's design is rumored to have been initiated by the Andalusian poet el-Saheli, circular plans had no precedent in the Maghreb. A tombstone with an inscribed epitaph dates this mosque to 1364. When Ibn Battuta visited Gao in 1353, during the reign of Kankan Musa, he reported it one of the largest and most beautiful cities in the Sudan. Although the Songhay leader Sonni Ali took power in 1464, it was not until Askia Mohammed came to power in 1493 that the city began to physically reflect the reclamation. 

In 1495, when Askia Mohammed invaded Diaga in the Massina, he took 500 Manding masons prisoner. Four hundred of them were sent to Gao, and another hundred were tasked with building the new city of Tendirma, where several massive earthen palace walls and the Friday mosque still remain. In addition to cultivating a building tradition that was to define the golden age of Sudanese architecture, Askia Mohammed's empire was also expanding. During his pilgrimage to Mecca, Askia was made Caliph of the Tekrur by the Sharif. As Prussin notes, "Western Sudanese monumental architecture, while perhaps a child of the Malian tradition, matured under Songhay sponsorship." Asia's tomb, whose construction is projected to have taken place between 1495 and the fall of the Songhay Empire to the Moroccans in 1591, eventually emerged as a sacred ancestral space. Under Askia Mohammed's successor, Askia al-Hajj II, the city was said to have been comprised of 7,626 permanent houses (excluding those of straw construction prominent on the outskirts of the city) with a population of about 75,000 people. 

Sources: 

Prussin, Labelle. Hatumere: Islamic Design in West Africa. Berkeley: University of California Press, 1986. 

Leary, A.H. The Development of Islamic Architecture in the Western Sudan. MA in African Studies dissertation. University of Birmingham, 1966. 

Saad, Elias N. Social History of Timbuktu: the role of Muslim scholars and notables 1400-1900. Cambridge: Cambridge University Press, 1983.