Although animistic beliefs remained potent in Sukhothai, King Ramkhamhaeng and his successors were all devout Buddhist rulers who made merit on a large scale. The major cities of the Sukhothai kingdom were, therefore, full of monasteries, many of which were splendid examples of Thai Buddhist architecture. Sukhothai adopted the Ceylonese school of Theravada Buddhism, beginning with King Ramkhamhaeng's invitation to Ceylonese monks to come over and purify Buddhism in his kingdom. This Ceylonese influence manifested itself not only in matters of doctrine but also in religious architecture. The bell-shaped stupa, so familiar in Thai religious architecture, was derived from Ceylonese models. Sukhothai style Buddha images are distinctive for their elegance and stylized beauty, and Sukhothai's artists introduced the graceful form of the "walking Buddha" into Buddhist sculpture.
Sukhothai's cultural importance in Thai history also derives from the fact that the Thai script evolved into a definite form during King Ramkhamhaeng's time, taking as its models the ancient Mon and Khmer scripts. Indeed, this remarkable king is credited with having invented the Thai script.
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.
The Sukhothai kingdom
did not die a quick death. Its decline lasted from the
mid-14th until the 15th century. In 1378, the Ayutthaya
King Borommaracha I subdued Sukhothai's frontier city
of Chakangrao [Kamphaengphet], and henceforth Sukhothai
became a tributary state of Ayutthaya. Sukhothai later
attempted to break loose from Ayutthaya but with no
real success, until in the 15th century it was incorporated
into the Ayutthaya kingdom as a province. The focus
of Thai history and politics now moved to the central
plains of present-day Thailand, where Ayutthaya was
establishing itself as a centralized state, its power
outstripping not only Sukhothai but also other neighbouring
states such as Suphannaphum and Lawo [Lopburi]