The City was nearly starving so an army inside the walls would have been impossible to feed. And Tywin would have been unable to leave to defend the West from Robb. But what I wanted to ask is that do you not really think that Cersei's report of how Tyrion made threats against Tommen were not vital in how he treated Tyrion later? I believe Tywin takes very protecting family very seriously and was genuinely outraged by this news. Tyrion comments how he seems to be usually angry. And Cersei told other unfavorable things about Tyrion as well and he did not defend himself. So I do not believe Tywin initially was planning on treating Tyrion the way he did or was worried about his success.