If I remember the book correctly, Cat seized one of Walder's sons or grandsons. I forget his given name, but I believe they called him Jinglebells. He was of diminished capacity and did not understand what was going on. Cat threatened to kill Jinglebells and Walder told her to go on because he had plenty of sons.
Walder's wife was a good subsitution. Cat's desperation when she realized what was happening chilled me in the books and the show.
The episode captured the brutality and the horror. I always felt so bad for the men outside at the feast burning to death in those tents.
I threw the book when I read that chapter and I was stunned when the credits rolled on the ep.
I think the TV show topped the books in brutality.
I was frustrated by Robb's choices in the books and on the show, but I was gutted by his death because of Cat's love and devotion.