Angkor Wat, a UNESCO heritage site, still shows the magnificance of the time over 1000 years ago. The major temple, Ankor Wat is surrounded by hundreds of smaller and still spectacular temples.
Angkor Wat was built for King Suriyavarman II in the 12th century as his state capital city. It was dedicated to Vishnu, the Hindu deity. During the 13th century the Mahayana Buddhist King Jayavarman VII build the Bayon, so you see the start of the Buddhist influence. The most impressive temple, for me, is Angkor Thom built in the 12th century.