What was the hard labor used to build?
The hard labor was slave labor used to build a temple to a female goddess, not a beautiful library or campus of education. It required umpteen hours of work to appease their gods and goddesses, as this practice was common throughout the ancient world, not just Corinth.
Les explains that the guide pointed out the massive effort to move building material from down below to up above. It was slave labor, umpteen hours of hard work, not to build some beautiful library or place of education, but a temple to a female goddess. This was all they lived for, working and sweating to somehow appease their gods and goddesses. This wasn't just Corinth; it was the whole ancient world. Along with the worship of these goddesses, which was even worse than the worship of the male gods, was gross immorality that was part and parcel of their everyday experience, all the way to the top. So the hard labor was used to build a temple to a female goddess.