However, it was most likely motivated by economic reasons. De Casparis then expands his theory, saying that the location of the kingdom in Central Java was less accessible than East Java. The Brantas river valley was considered to be a strategic location, as the river provides easy access to reach ports on East Java's north coast and Java Sea, strategic for the control of maritime trade routes to the eastern parts of archipelago, being especially vital for control of the Maluku spice trade. This is in contrast with Mataram's Kedu and Kewu Plain that relatively isolated from the north coast of Central Java. Despite its fertility, ideal for rice agricultural kingdom, the Mataram Plain is quite isolated, its northern borders are protected by natural barrier of Merapi, Merbabu, Sumbing, Sundoro, Dieng and Ungaran volcanoes. Ideal for inward-looking agricultural polity, but insufficient to develop a maritime trading kingdom.