Complex sentence: Despite the scarcity of resources in Siam, coastal ports and cities and those at the river mouth were early economic centers which welcomed merchants from Persia, Arab countries, India and China.
Split into simpler sentences: Despite the scarcity of resources in Siam, the coastal ports and cities, as well as those at the river mouth, were among the early economic centers. These centers welcomed foreign merchants from Persia, Arab countries, India and China.