@soumahr
Well, it is set in some kind of European middle age setting
If you know a little bit about European history, you know that Europe did not have 'slaves' by name. They owned land and if ppl wanted to live they had to get permission to work on that land as farmers or something else. At the same time they 'sold' themselves to that lord. They could not marry without the lords permission. And the lord had the right of the first night. Therefore it was not seldom that the bride was raped on her wedding night by that lord and even conceived his child. The lord could do whatever he wanted while still pretending they were free as he had the complete juristriction over them. Therefore they could have also been called slaves.
Even in todays times slavery exists in different country's.
I don't support slavery. And sure in this setting its ok.