Reizoku Joufu Shuuyoujo ~The Witch of Belladonna~

During a time of war in a certain country, a beautiful forest surrounds a castle that houses over 300 prisoners of war. That is where the beautiful military physician lieutenant colonel Rosenberg has been transferred. Through a particular kind of torture, her job is to extract military secrets from men. In a space that exceeds normality, the anguish and pleasure leave those fine men seeking more.

(from mangaupdates)