He goes to her cell
From Create Your Own Story
Robb decided to go for the less glamorous option. He wanted to see the Queen in rags, put in her rightful place. Asking a guard for directions, he was about to descend to the Red Keep's dungeons when he was stopped by Mira Forester. Apparently, two ravens had arrived for him, both seemingly important.
Business before pleasure, Robb thought, retreating to his room.
The first letter was from Lord Forrester. He offered the hand of his daughter Talia in marriage. It was to be expected, every Northern Lord with an unmarried daughter would send him a letter like that or visit Winterfell in the next weeks. Not that Robb would complain!
The second letter was from his uncle Edmure. He wrote that Lord Frey had died in his sleep and Ryman Frey was new Lord of the Twins, albeit heavily contested by various brothers and nephews and cousins and whatnot. While the contract of marrying a Frey girl was still valid, Edmure implied that Robb should not feel too bound by it - the Freys were suddenly a lot less powerful and Ryman did apparently not care too much about his grandfather's plans. Robb smiled. Everything had gone a lot better than expected, now he was even free to choose a wife again. He read the rest of the letter. Edmure would send Roslin Frey, arguably the prettiest of the lot, to Winterfell.
Robb quickly wrote a response to Lord Forrester, inviting him and his family to Winterfell so he could meet his daughter. Let's gather all the girls at home so I can take my pick, Robb thought nefariously and imagined dozens of nubile girls lining up naked in the Great Hall before he banished the picture from his mind.
With no more letters to sign, Robb excitedly started for the dungeons. The lower levels of the Red Keep were a maze, and he had to ask for directions twice. The trip was well worth it, though, as soon enough he laid eyes upon the former Queen of Westeros.
She was lying in a pile of hay, with no signs of comfort whatsoever. Her hair was a mess, ungroomed and dirty, unlike the usually pristine Lady Robb had met at Winterfell. Her face was simiarly dirty, and so were her legs and arms. However, even covered in dirt and wearing rags, she was still attractive.
Upon hearing him enter, Cersei spun her head in his direction, staring daggers at him. Desperately trying to hold unto her pride, she stood up, her best attempt to appear stronger than the Lord of Winterfell. I'll fix that soon enough, Robb thought.
He walked around her, taking in every inch of her. Cersei followed him with hostile eyes and thin lips. "Lady Cersei", Robb said slowly, coming to a halt in front of her, standing uncomfortably close. "You killed my father."
She didn't break eye contact. "He was a traitor. He tried to usurp the throne." Robb, suddenly angry, backhanded her across the face. Cersei was throne to the floor, landing on her stomach.
"HE WAS A GOOD MAN!" Robb yelled, all his anger over his father coming back, "And you executed him! And now," Robb eyed her full ass, "You must be punished"
How does Robb punish her?