From the 6th century onward they became more important as there was an increase in trade ( mainly with Arab merchants ) , population growth , and further centralized urbanization . So the Swahili City-States developed . They evolved gradually from the 6th century onward to accommodate for an increase in trade ( mainly with Arab merchants ) , population growth , and further centralised urbanisation ; developing into what would later become known as the Swahili City-States .