Recently we visited the temples of Angkor Wat. Nothing prepares you for the size and scale of the temples. Built during the golden era of Angkor from AD 802 to 1432 when the God Kings strove to build the biggest and the best (now we would build a skyscraper).
During this period the ancient Khmer capital had around 1 million people (London had around 50,000 by contrast). As well as temples there are vast Baray's or reservoirs to supply water to grow rice.
Angkor Wat is of course the most famous. It is Hindu in orientation whereas later all are Buddhist. The stonework is exquisite. Not even hunderds of years of ageing and abandonment take away the wonder. There are literally hundreds of ruins - we covered about a dozen in 3 days.
If you love history you must go there before it becomes overpopular with tourists. Its ok right now - and the rainy season is perfect for creating a moody atmosphere.