Zelda Informer: "At the halfway mark of Twilight Princess, after Zelda saves Midna, our heroes teleport outside of the castle in time to watch a massive barrier form around the castle. Upon returning to the town later, they find the place...absolutely the same as ever before. The huge barrier, plainly visible, isn't even mentioned except by a random Goron; the rest of the townsfolk, either blind or stupid, continue on with their lives completely unchanged.
So the problem with Hyrule Castle's barrier is twofold: first, the citizens don't react to it; second, Ganondorf does nothing while inside it. Even if he couldn't leave the castle for some reason, he still had monsters inside the barrier that could have been sent out. Fixing the former issue would have hardly any effect on the storyline, but the latter...that one has some major implications that simply cannot be ignored.
Because quite frankly, the only believable action for Ganondorf to take is to attack those citizens.
Picture it: Link and Midna warp out of the castle and see the barrier appear, only this time the townsfolk take note of it too – and not just because of the barrier, but because of the hordes of monsters rushing out of it."