It was definitely Harrenhal. Got some of the backstory from the GoT wiki:
http://gameofthrones.wikia.com/wiki/Harrenhal
http://gameofthrones.wikia.com/wiki/Gregor_Clegane
So I guess there was no battle there. Robb was marching there with the intention of seizing it, but the previous lord, Ser Gregor Clegane, abandoned it. Before leaving, he killed all the prisoners.
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You don't know it's Selmy until the 4th or 6th book, can't remember which.
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That's pretty cool. I could see that being an awesome reveal when you finally find out. Makes sense that they had to change it for tv though.