The Return of the King is the third novel of the Lord of the Rings Trilogy. Tolkien began this trilogy well after the "world" of Middle Earth had been created. Several of the characters presented in this novel were first presented in The Hobbit. The Silmarillion, Tolkien's major mythological masterpiece, also presents an extensive history and language system that was already in place (though The Silmarillion was published posthumously). Much like the stories of Greek mythology, these stories present a history of the different "ages" of the world. The trilogy takes place in the Third Age.
The Middle Earth is a complicated mythology, out of which the story of Lord of the Rings grows and on which it is deeply dependent. Besides this mythological aspect, some critics also see Tolkien's work as a reflection on the horrors and politics of the Second World War. The delineation between the East and the West is also revealing. While the reader should avoid taking these insights to the extreme, this is an added context that makes the work more complete. It is well worth noting that this storyline was invented before the war, but it must have altered Tolkien's thought.
Finally, we should note Tolkien's role as an academic at Oxford University. His study of ancient and Anglo-Saxon languages certainly played a role in his invention of language systems. We can also look at the importance of saying certain words or not saying other words. Tolkien's Christian perspective also plays a role in his organization of Middle Earth. He and his colleague, C.S. Lewis, both create worlds that seek to understand the role of the individual's free will and the ability to choose between good and evil. The power-play between the forces of freedom and the dark lord, Sauron, simulate the battle between good and evil on earth.