King Si Inthrathit and King Ramkhamhaeng were both warrior kings and extended their territories far and wide. Their successors, however, could not maintain such a far-flung empire. Some of these later kings were more remarkable for their religious piety and extensive building activities than for their warlike exploits. An example of this type of Buddhist ruler was King Mahathammaracha Lithai, believed to have been the compiler of the Tribhumikatha, an early Thai book on the Buddhist universe or cosmos. The political decline of Sukhothai was, however, not wholly owing to deficiencies in leadership. Rather it resulted from the emergence of strong Thai states further south, whose political and economic power began to challenge Sukhothai during the latter half of the 14th century. These southern states, especially Ayutthaya, were able to deny Sukhothai access to the area.