In the 19th century Stone Town flourished as a trading centre. It was especially renowned for the commerce of spices (mostly cloves) and slaves. Around middle of the century, the sultanate had a close relationship with the British; David Livingstone, for example, is known to have stayed in Stone Town in 1866 while he was preparing his final expedition into the interior of East Africa. In the same period, several immigrant communities from Oman, Persia and India formed as a consequence of the town's intense commercial activity. The Sultan of Zanzibar encouraged immigration of foreign traders who became very wealthy and settled in the city who brought diversity to the city's architecture.