I was just rereading the CBC piece you linked in the response to the Dornish nobles ask and it got me to wondering. What was Renly’s plan for crossing the Blackwater? Since the Redwynes aren’t supporting him because of the hostages, he doesn’t have any naval power of his own, right? Relatedly, why would he build siege engines at Bitterbridge and haul them all the way up to King’s Landing rather than just building them there?

My guess is that he wanted to ford the river further upstream where he wouldn’t have to deal with the royal navy at all, and attack the city from the north. As for hauling them, yes, that is incredibly slow but on the other hand it means the enemy can’t burn or cut down the trees to deny you building materials, and Renly wasn’t in a hurry. Indeed, it may have been the case that he thought that a sufficient show of preparation might convince Tyrion to surrender peacefully.