Ok I have an idea, and it's something i'll be using for my castle walls.
Find a good base card colour for the walls.. The grey is fine.. then get a set of acrylic paints and mix black and white together to make a darker dull grey (but not quite black).
Get a sponge and cut out the sponge into two parts to make two bricks.. when you dip the sponge into the paint mix it absorbs and you can press the effect onto the paper as you like.
Wait for it to dry.. I have a picture here of the castle, throne room (work in progress)
The walls look really good. Maybe the room or bricks would be too big for your prison or dungeon. But if its a dungeon.. I was thinking castle.. and usually they didnt have standard sized bricks.. they were usually huge stones..
