That's why I said access across the river, not bridge. There are certain things that can be done to keep boats from reaching the shore. There are certain things that can be done to shore up the shore. There are certain things that can make it impossible to cross a river except at a very narrow crossing, thus making it 'easy pickins' for the defenders. Again there are tons of military mistakes in both the books and the movies. However, having the giant eagles of Manwe fly the ring into Mt. Doom doesn't make for a good story. One of my biggest
is the whole ring issue, why couldn't Sauron just as easily forge a new ring to rule over the one ring? Plus the wizards had knowledge of black-powder, I am pretty sure black-powder bombs or guns might just make it easier to win the war. But Tolkien hated that technology so it wasn't used, he preferred the killing in LoTR to be done with knives, not guns.