that the original structure was Portuguese, while the present form of the fort is of typical Omani forts. The word Gereza means prison in Swahili, possibly indicating the use of the fort as an Omani slave holding building during the late 18th century to late 19th century after the collapse of the Swahili civilization after the arrival of the Portuguese in late 16th century. Controversies A lot of Kilwa's history has been written and politicized by Omani and European colonial administrators in the 19th century to justify the occupation and domination of East Africans over the centuries. This view assumes that cultural innovation and historical development in Africa can only come from outside. Hence there has been a lot of contradictory evidence on the origins and the role of foreign immigrants in Kilwa's history. According to local oral tradition, in the 11th century the island of Kilwa Kisiwani was sold to Ali bin Hasan, son of the "King" of Shiraz, in Persia. Another tradition relates that his mother was Somali. Ali bin Al-Hasan is credited with founding the island city and with marrying the daughter of the local king. Though he was credited with the founding, he had arrived at an already inhabited area. He did, however, come to power and is credited with fortifying the city and increasing trade. Tradition also relates that it was the child of this union who founded the Kilwa Sultanate. Archaeological and documentary research has revealed that over the next few centuries, Kilwa grew to be a substantial city and the leading commercial entrepôt on the southern half of the Swahili Coast (roughly from the present Tanzanian-Kenya border southward to the mouth of the Zambezi River), trading extensively with states of the Southeast African hinterland as far as Zimbabwe. Trade was mainly in gold, iron, ivory and other animal products of the interior for beads, textiles, jewelry, porcelain and spices from Asia. On the contrary, there is no evidence of Shirazi-based Shia Islam in Kilwa and the entire East African coast. By the 12th century, under the rule of the Abu'-Mawahib dynasty, Kilwa had become the most powerful city on the Swahili Coast. At the zenith of its power in the 15th century, the Kilwa Sultanate claimed authority over the city-states of Malindi, Mvita (Mombasa), Pemba Island, Zanzibar, Mafia Island, Comoro, Sofala and the trading posts across the channel on Madagascar. Ibn Battuta recorded