The Khmer Empire effectively dominated all Mainland Southeast Asia from the early 9th until the 15th century, during which time they developed a sophisticated monumental architecture of most exquisite expression and mastery of composition at Angkor. In modern-day Vietnam, the kingdoms of Đại Việt and Champa became rivals to the Khmer Empire in the region. The distinct culture of the kingdom of Dvaravati first appeares in records around the 6th century. By the 10th century, Dvaravati had come under the influence of the Khmer Empire and by the 12th century Thai tribes had conquered the Chao Phraya River valley of modern-day central Thailand and established the local Sukhothai Kingdom in the 13th century and the Ayutthaya Kingdom in the 14th century.