And what were you doing just hours before, hmm? Invading our lands like a barbarian? I give you one last chance to tell me everyting thing you know before I become "uncivilised"
If he doesn't know anything, which is extremely likely. just say something like "well you have told me enough to buy your life" and toss him out. Then get a slightly higher ranking guy and do the same to him. The last guy to be interrogated will be the knight.