This book would have been read to me in the late 70’s as a child. A man is locked in a stone tower or room (maybe in a castle) with only a small window at the very top. He must find a way out without opening the door. He ends up covering himself in red dots so the guards think he’s ill and take him out, he has found a way out without having opened the door. It was like a puzzle or riddle of sorts. I believe the man had red hair or a red beard.