A Swahili culture developed in the towns, notably in Pate, Malindi, and Mombasa. The impact of Arabic and Persian traders and immigrants on the Swahili culture remains controversial. During the Middle Ages, the East African Swahili coast [including Zanzibar] was a wealthy and advanced region, which consisted of many autonomous merchant cities. Wealth flowed into the cities via the Africans' roles as intermediaries and facilitators of Indian, Persian, Arab, Indonesian, Malaysian, African and Chinese merchants. All of these peoples enriched the Swahili culture to some degree. The Swahili culture developed its own written language; the language incorporated elements from different civilisations, with Arabic as its strongest quality. Some Arab settlers were rich merchants who, because of their wealth, gained power—sometimes as rulers of coastal cities. Portuguese explorers appeared on the East African coast at the end of the 15th century. The Portuguese did not intend to found settlements, but to establish naval bases that would give Portugal control over the Indian Ocean. After decades of small-scale conflict, Arabs from Oman defeated the Portuguese in Kenya. The Portuguese became the first Europeans to explore the region of current-day Kenya: Vasco da Gama visited Mombasa in April 1498. Da Gama's voyage successfully reached India (May 1498), and this initiated direct maritime Portuguese trade links with South Asia, thus challenging older trading networks over mixed land and sea routes, such as the spice-trade routes that utilised the Persian Gulf, Red Sea and caravans to reach the eastern Mediterranean. (The Republic of Venice had gained control over much of the trade between Europe and Asia. Especially after the Ottoman Turks captured Constantinople in 1453, Turkish control of the eastern Mediterranean inhibited the use of traditional land-routes between Europe and India. Portugal hoped to use the sea route pioneered by da Gama to bypass political, monopolistic and tariff barriers.) Portuguese rule in East Africa focused mainly on a coastal strip centred in Mombasa. The Portuguese presence in East Africa officially began after 1505, when a naval force under the command of Dom Francisco de Almeida conquered Kilwa, an island located in the south-east of present-day Tanzania. The Portuguese presence in East Africa served the purpose of controlling trade within the Indian Ocean and securing the sea routes linking Europe and Asia. Portuguese naval vessels disrupted the commerce of Portugal's enemies within the western Indian Ocean, and the Portuguese demanded high tariffs on items transported through the area, given their strategic control of ports and of shipping lanes.