comparison

Comparison from the source material
The screenwriters did not originally script The Two Towers as its own film: instead parts of it were the conclusion to The Fellowship of the Ring, the first of two planned films under Miramax.[4] However, as the two films became a trilogy under New Line, Jackson, Walsh and Boyens shuffled their scripts. The Two Towers is known as the most difficult of the Rings films to make, having neither a clear beginning nor end to focus the script.[5] Nonetheless, they had a clear decision with making the Battle of Helm's Deep the climax, a decision affecting the whole story's moods and style.
The most notable difference between the book and the film is the structure. Tolkien's The Two Towers is split into two parts; one follows the war in Rohan, while the other focuses on the journey of Frodo and Sam. The film omits the opening of the book, the death of Boromir, which was used as a linear climax at the end of The Fellowship of the Ring. Also, the film climaxes with the Battle of Helm's Deep, while the book ends with the Fellowship going to Isengard and Frodo's confrontation with Shelob, scenes which were left for film adaptation of The Return of the King. This was done partly to fit more closely the timeline indicated by the book.
One notable change in plotting is that in the film Théoden is literally possessed by Saruman, but in the book he is simply depressed and deluded by Wormtongue. Afterwards, in the film, Théoden is still unsure of what to do, and flees to Helm's Deep. In the book he rides out to war, only ending up besieged when he considers helping Erkenbrand. Erkenbrand does not exist in the films: his character is combined with Éomer as the Rohirrim general who arrives with Gandalf at the film's end. Éomer himself is present during the entire battle in the book.
On the way to Helm's Deep, the refugees from Edoras are attacked by Wargs. The scene is possibly inspired by one in the book cut from The Fellowship of the Ring where it is the Fellowship who battle them. Here, a new subplot is created where Aragorn falls over a cliff, and is assumed to be dead; Jackson added it to create tension.[6] This scene also resonates with a new subplot regarding Arwen, where she decides to leave Middle-earth after losing hope in the long-term possibilities of her love. In the book, Arwen's role is primarily recorded in the Appendices, and she is never depicted as considering such an act.
A larger change was originally planned: Arwen and Elrond would visit Galadriel, and Arwen would accompany an army of Elves to Helm's Deep to fight alongside Aragorn. During shooting, the script changed, both from writers coming up with better ideas to show the romance, as well as poor fan reaction.[5][7] The new scene of Arwen leaving for the West was created, and the conversation scene remains, edited to a telepathic one.[8] Nonetheless, one major change (already filmed) remained that couldn't be reverted: the Elven warriors fighting at Helm's Deep. However, Jackson looks upon the change as highly romantic and stirring that the Elves return to fight for the future of Middle-earth.[5]
Another change is the fact Treebeard does not immediately decide to go to war. This adds to the tension, and Boyens describes it as making Merry and Pippin "more than luggage".[6] Here the Hobbits make Treebeard see the full destruction, prompting his anger and decision to act. Another structural change is that the Hobbits meet Gandalf the White early on, possibly explaining why the Hobbits don't react to his return when they meet him again following the destruction of Isengard. This was explained in the book by Gandalf arriving at Isengard in the middle of the night to talk to Treebeard.
The filmmakers' decision to leave Shelob for the third film meant that Faramir had to become an obstacle for Frodo and Sam.[5] In the book, Faramir (like Aragorn) quickly recognizes the Ring as a danger and a temptation, and does not hesitate long before letting Frodo and Sam go. In the film, Faramir first decides that the Ring shall go to Gondor and his father, as a way to prove Faramir's worth compared to his elder brother Boromir. In the film, Faramir takes Frodo, Sam and the Ring to the Battle of Osgiliath - they do not go there in the book. Jackson winks to readers with Sam's line, "By all rights we shouldn't even be here, but we are." After seeing how strongly the Ring affects Frodo during the Nazgûl attack, Faramir changes his mind and lets them go. These changes dilute (or at least reshape) the book's strong contrast between Faramir and Boromir, who in The Fellowship of the Ring attempted to take the Ring for himself. On the other hand (which can be seen only in the extended version of the film), it is actually their father Denethor, who wants the ring and urges Boromir to get it, while Faramir only wants to prove that he also deserves his father's love. Boyens contends these plot changes were needed to keep the Ring menacing. Wenham himself, despite having not read the book, found the original character "dramatically dead".[5]
Finally, the meaning of the title itself, 'The Two Towers', was changed. While Tolkien considered several possible sets of towers[9] he eventually created a final cover illustration[10] and wrote[11] a note included at the end of The Fellowship of the Ring which identified them as Minas Morgul and Orthanc. Jackson's movie names them as Orthanc and Barad-dûr, symbolic of an evil alliance out to destroy Men that forms the film's plot point.
source:Wikipedia