In the mission to gather the members of the former Lodge of Sorceress, it fell to Yennefer to bring Margarita Laux-Antille to Novigrad. Yennefer's old friend had been captured by the witch hunters and was being kept in Deireadh, the best-guarded prison this side of the Yaruga.
Geralt and Yennefer thus had a hard endeavor in front of them - they would have to break into the sealed compound and sneak Margarita out. Good thing Yen had something even better than a file in a carrot cake: contact information for the one man who had managed to escape Deireadh. Escaping seemed to be in his blood, for he attempted to make another getaway at the sight of Yennefer and Geralt, but eventually they caught him and sat down for a chat. He told them how to outfox the guards and enter the prison unnoticed. With no time to waste, Geralt and Yennefer raced off to Margarita's rescue.
With their powers combined, Yennefer and Geralt freed Margarita from the most formidable prison in the North. Rita was in a very sorry state, but the knowledge that she was free and would not perish on a pyre fortified her strength. Yennefer showered her long-time friend with tender care, and everyone hoped for a swift recovery.
If you helped Triss Merigold earlier in the main questline and completed Now or Never, Zoltan Chivay will be unable to help supply alcohol to Geralt in the quest due to the danger of the witch hunters diverting their attention from mages (who have now fled from Redanian rule) to non-humans.
Once you have the key you can search the cells for a letter and one of the rooms above the courtyard where you encounter several guards has a List of Prisoners. The prisoners listed are friends or associates of the developers.
Opening the cells will free the other unfortunate prisoners. Once you free Margarita, you lose the key and cannot open any of the cell doors.