Robb should just march on Casterly Rock now, whos gonna stop him?
Eh, I don't think that's going to happen. As far as I can remember, we haven't seen Casterly Rock yet, but I imagine that it's got some sort of defenses, so I'm guessing Robb won't be able to just walk over and take it. Even if he does, he's got the combined Lannister/Tyrell force to contend with now, which would make actually holding onto Casterly Rock pretty tricky.