Thank you so much, @fitinfun! :)

That's a very interesting question! Here's what I found about it:

The building of Angkor Wat is likely to have necessitated some 300,000 workers, which included architects, construction workers, masons, sculptors and the servants to feed these workers.
The Khmer rulers made use of a vast force of slave labour, there were probably thousands of slaves. Each new temple was made possible by the 750,000 people estimated to have lived at Angkor during the 12th and 13th centuries. Angkor Wat took approximately 34 years to build, resulting in many slaves dying in the process from being overworked.
Slaves were either captured during war, or were indigenous Mon-Khmer Tribespeople, who had been taken from the eastern highlands of the Khmer Empire. And there were also debt slaves, whom were people enslaved until they could pay off their dept.
Most families had 100 or more of them [slaves], a few had 10 or 20; and only the poorest peasants didn't own at least a single slave.

I suppose that slaves were used for tasks that didn't require skill, like hauling the stone blocks and feeding those skilled workers... what do you think?