How to open Rattigan's cage in No Rest for the Wicked

When you try to talk to Rattigan on a mission, he will refuse to communicate further until you release him. In other words, you must figure out how to free him from the cage. To do this, go south of the cage until you see a spiral staircase. Climb it, and you'll hear a guard talking about another guard who fell asleep at the fountain. The words "fountain" and "key" are highlighted in the text dialog, giving you a good idea of how to free Rattigan.

You need to head to the fountain. The image below shows the exact location of the fountain you need to get to in order to get the key to Rattigan's cage.

Once at the fountain, you will notice a guard sleeping on the ground. Interact with him to get the cell key. Go back and free Rattigan.After freeing Rattigan from the cage, he will decide to help you and you can now continue with the side quest. He will explain that Sacramenta is being prepared for an attack and that he can help stop it. He will suggest meeting at the entrance to the Communion sewer. Follow to the new marker to continue the quest.