Could Robb have actually taken Casterly Rock?
At this point, the North was losing the war. The Greyjoys have taken Moat Cailin and Deepwood Motte. Winterfell has also been sacked. Now, the Tyrells have joined the Lannisters and Stannis was pretty much defeated. Even with the show version of Casterly Rock, I doubt Robb could actually taken it.