After getting K.O.'ed by Tiggy, you wind up in an unfamiliar cell somewhere in an old keep. Look around -- the guard seems to be asleep; an opportune time to make an exit.
You'll probably notice right away that one square in the cell is different from the others, and if you walk onto it, you will be presented with a dialog that indicates the boards are squeaky in that area. You should stand beside that square (not directly on it) and then activate it. This opens a route to a chamber below the cells.
