The Game Facility was probably built by slaves.

The number of people required to build the facility would be much higher than the number of guards, and I doubt the games had the same clear and easy access to people willing to work for them for dirty money. The culture of the games hadn’t even been established yet.

The most plausible explanation , imo , is trafficked slaves. Perhaps given some sort of “pay” in order for Il-Nam to justify his evil as “just the way of the world”. Perhaps they were even simply executed after it was all said and done and the money went to their relatives.

Anyway, kind of a random and morbid thought and I doubt the exact methods were ever considered, but still an interesting idea.