Have you ever visited an honest-to-goodness medieval castle? They're often filled with medieval graffitti.
e.g. http://www.flickr.com/photos/hauxtongill/2748365297/
From my understanding, prisoners and captives did not get the same kind of cells and treatment. Prisoners being those arrested for crimes (commoners), and captives being other nobles captured in battle and detained for ransom. I would expect graffiti from the former, but not the latter.
