Upon closer inspection of the bars, it seems that the large silvery pattern on the north wall has tendrils that flow onto the east and west walls, and those tendrils break up and touch each and every place where the bars enter stone. Oddly there seems to be no door to these cells, just bars of metal stuck into the stone, as if the prisoners were sealed in there.
The InterrogationI am assuming that Castille's 16 and 9 on Sense Motive affect the western prisoner and the eastern prisoner respectively, as well as Kaspar's 16 and 22. Thus, Castille and Kaspar both got a 16 on Sense Motive-ing the western prisoner, but Kaspar's fine intuition on the eastern prisoner shines far better than Castille's. (For simplicity's sake I will call the prisoners WP and EP.)
WP seems to be telling the truth, so far as you can tell. Your 16 Sense Motive should catch all but the very experienced of liars. EP on the other hand seems a little fishy to Kaspar. Kaspar gets the feeling that EP is bluffing.
Kaspar approached the prisoner on the right, which would be the east, and thus he approached EP, who he thinks is lying about something. His challenges to EP are met as follows.
EP: "I am the one telling the truth! I am Joseph Woodehouse, refugee from Cyre, and minister to the downtrodden. I served my village of Meadowshine for long years during the war, providing -"
WP: "Oh stop it, stop claiming my village and my name, just stop!"
Kaspar shoots WP a black look and WP sighs.
EP: "As I was saying, before this evil thing interrupted me, I fled my country on the Day of Mourning that destroyed Cyre, and I wandered to Breland. In Breland I was ambushed by dark clerics who snared me with webs made of shadow. I dreamt horrible dreams in captivity, awakening only briefly to find myself in Aundair, and then again to watch them crave my likeness onto that undead thing! Now I sit, taunted by a creature from beyond the grave that mocks me with my own voice and claims to be me! The feed me moldy bread and filthy water and bring that - that abomination - cups of blood from their sacrificial chamber -"
WP: "SHUT! UP! I can't listen to you anymore! I've never been the Breland -"
EP: "No, you were assembled here in Aundair!"
WP: "He's a changeling, a shapeshifter, one fo the cultists, you don't believe his lies, do you?"
EP: "You will be found out, foul creature, have the decency finally, the simple decency -"
WP: "How dare you speak of decency?"
The bickering begins immediately, and in the insults you hear that EP claims not to have married and WP claims that his wife died in Cyre on the Day of Mourning. Both claim to be 52 years of age, and they look it. EP claims to have an earliest memory of an apple harvest, for WP it is his mother singing.
Castille's ExaminationCastille frowns, looking at the loops and swirls, as memories of his mentoring in magic come back to him...
It's a lock. The silvery metal is painted on, a mithril powder mixed with some sticky paste, probably the blood of a fiend, and its purpose is to keep the wall ahead of Castille sealed shut. What the key is for Castilel does not know as neither the prisoners' cells nor the wall ahead of him contain a lock. But the silvery design is definitely holding something behind the wall. And it seems powered in part by the tendrils that reach onto the east and west walls.