Helm's Deep
In the second volume of "The Lord of the Rings" by J.R.R. Tolkien, the forces of Saruman assaulting the fortress of Helm's Deep use two tree trunks as improvised rams. In the third volume, Sauron's army uses a battering ram named Grond to assault the Great Gate of Minas Tirith. It was 100 feet long and capped with an iron wolf's head.