How did prosperous trading cities grow up along africa’s east coast?

"Africa's East coast had drawn overseas traders from early times. The main reason was the influence of the Indian Ocean and its monsoon winds. Between November and March, the monsoons blew southwest from the coast of India toward Africa. From April to October, the monsoons reversed and blew northeast toward India and the Persian Gulf. Over time, sailors learned to take advantage of these winds to move around the Indian Ocean. A trade network soon developed that linked East Africa with Persia, Arabia, India, and even Southeast Asia. Market towns sprang up along the East African coast to take advantage of that trade. Arab traders called this East African coastal region the land of Zanj and used the monsoon winds to visit these port towns. By AD 1100 several of these coastal market towns had grown into wealthy and thriving city-states. The main city-states were Mogadishu, Mom-basa, Kilwa, and Sofala.
The coastal city-states linked merchants from overseas with traders from Africa's inte-rior. In the city-states' markets, merchants from overseas sold luxury items such as glassware, East Asian porcelain, and silk and cotton from China and India. In exchange for these manufactured luxury items, foreign merchants obtained raw materials from Africa's interior. These goods included coconut oil, copper, leopard skins, and shells. African ivory, which was highly prized in many countries, was another valuable trade item along with gold from southern Africa. In addition, enslaved Africans captured in the interior were exported through the coastal city-states to slave markets in Arabia, Persia, and India. These enslaved Africans were then sent to regions across Asia, many to work as domestic servants. The trade of enslaved Africans would later increase substantially after Europeans began coming to Africa. Many of the enslaved Africans in this later European slave trade would be exported to the Americas. Trade along East Africa's coast reached its peak during the 1300s and 1400s. By this time, Kilwa had become the wealthiest and most powerful of the coastal city-states. Kilwa became a prosperous trading center because it was located at the southernmost point on East Africa's coast that a ship could reach in a single monsoon season. All African goods from south of Kilwa, therefore, were exported from Kilwa's market. Kilwa's power increased further in the late 1200s when it gained control of Sofala, through which much gold was exported"