Tumbatu were the dominant centers of exchange. Zanzibar was just one of the many autonomous city-states that dotted the East African littoral. These towns grew in wealth as the Swahili people served as intermediaries and facilitators to merchants and traders. This interaction between Central African and Indian Ocean cultures contributed in part to the evolution of the Swahili culture, which developed an Arabic-script literary tradition. Although a Bantu language, the Swahili language as a consequence today includes some borrowed elements, particularly loanwords from Arabic, though this was mostly a 19th-century phenomenon with the growth of Omani hegemony. Many foreign traders from Africa and Asia married into wealthy patrician families on Zanzibar. Particularly Asian men, who "wintered" on the coast for up to six months because of the prevailing monsoon wind patterns, married East African women. Since most Asian traders were Muslim, their children inherited their paternal ethnic identity, though East African matrilineal traditions remained key. Portuguese colonization Vasco da Gama's visit in 1498 marked the beginning of European influence. In 1503 or 1504, Zanzibar became part of the Portuguese Empire when Captain Ruy Lourenço Ravasco Marques landed and demanded and received tribute from the sultan, in exchange for peace. Zanzibar remained a possession of Portugal for almost two centuries. It initially became part of the Portuguese province of Arabia and Ethiopia and was administered by a governor general. Around 1571, Zanzibar became part of the western division of the Portuguese empire and was administered from Mozambique. It appears, however, that the Portuguese did not closely administer Zanzibar. The first English ship to visit Unguja, the Edward Bonaventure in 1591, found that there was no Portuguese fort or garrison. The extent of their occupation was a trade depot where produce was purchased and collected for shipment to Mozambique. "In other respects, the affairs of the island were managed by the local 'king', the predecessor of the Mwinyi Mkuu of Dunga." This hands-off approach ended when Portugal established a fort on Pemba Island around 1635 in response to the Sultan of Mombasa's slaughter of Portuguese residents several years earlier. Portugal had long considered Pemba to be a troublesome launching point for rebellions in Mombasa against Portuguese rule. The precise origins of the sultans of Unguja are uncertain. However, their capital at Unguja Ukuu is believed to have been an extensive town. Possibly constructed by locals, it was composed mainly of perishable materials.