If i remember the spoilers i read right, it's technically a harem in the more traditional sense, aka none of the women actually sleep with the archduke (harem's didn't always mean sexual favors and such, in some ancient cultures a harem could be similar to a women's boarding house or a convent), it's a home for young women the archduke saved, i'm not sure why the FL isn't in there but it might be consideration for her past and to avoid any kind of misunderstandings.