Short answers, pulled from the story.
The Khmer Empire is conventionally dated from 802 AD, when Jayavarman II declared himself chakravartin on Mount Mahendraparvata, to 1431 AD, when King Ponhea Yat abandoned Angkor and moved to the Phnom Penh area. The period is known to historians as the Angkor period.
Angkor Wat was built under King Suryavarman II, who reigned from 1113 to 1150, over a period of 37 years. It was dedicated to the god Vishnu and also served to honour Suryavarman II in his posthumous identity as Vishnu; its Sanskrit name, Vara Vishnuloka, means the realm of Vishnu.
Scholars identify multiple causes: chronic internal power struggles among Khmer princes, the shift from Hinduism to Theravada Buddhism which undermined the divine authority of kings, ecological breakdown of the hydraulic canal system caused by deforestation and alternating droughts and floods, and repeated military pressure from the Siamese Ayutthaya Kingdom, which captured Angkor in 1353 and again in 1394.
The Greater Angkor Region had a population of approximately 700,000 to 900,000 at its peak in the 13th century CE, making it one of the most populous cities of the medieval world.
Jayavarman VII reigned from 1181 to 1219 and is generally considered Cambodia's greatest king. He built the new capital Angkor Thom with the Bayon as its state temple, constructed the temples Ta Prohm and Preah Khan, established an empire-wide road network with rest-houses, and built a total of 102 hospitals across his realm.
Zhou Daguan was a Chinese diplomat sent by the Yuan dynasty emperor Temur Khan who arrived in Angkor in August 1296 and stayed until July 1297. His account, The Customs of Cambodia, is one of the most important surviving sources on Khmer daily life, culture, and society, and records details including the marketplace, royal processions, housing, textiles, and the appearance of the Bayon's gold-covered towers.