The decline continued through a transitional period of approximately 100 years followed by the Middle Period of Cambodian history, also called the Post-Angkor Period, beginning in the mid 15th century. Although the Hindu cults had by then been all but replaced, the monument sites at the old capital remained an important spiritual centre.^[10] Yet since the mid 15th century the core population steadily moved to the east and – with brief exceptions – settled at the confluence of the Mekong and Tonle Sap rivers at Chaktomuk, Longvek and Oudong.^[11][12]

Maritime trade was the basis for a very prosperous 16th century. But, as a result foreigners – Muslim Malays and Cham, Christian European adventurers and missionaries – increasingly disturbed and influenced government affairs. Ambiguous fortunes, a robust economy on the one hand and a disturbed culture and compromised royalty on the other were constant features of the Longvek era.^[13][14]

By the 15th century, the Khmers’ traditional neighbours, the Mon people in the west and the Cham people in the east had gradually been pushed aside or replaced by the resilient Siamese/Thai and Annamese/Vietnamese, respectively.^[15] These powers had perceived, understood and increasingly followed the imperative of controlling the lower Mekong basin as the key to control all Indochina. A weak Khmer kingdom only encouraged the strategists in Ayutthaya (later in Bangkok) and in Huế. Attacks on and conquests of Khmer royal residences left sovereigns without a ceremonial and legitimate power base.^[16][17] Interference in succession and marriage policies added to the decay of royal prestige. Oudong was established in 1601 as the last royal residence of the Middle Period.^[18]