Feudal Britain
The novel is set just before the birth of the British nation, mythically speaking. Arthur is the first true king of Britain, and so the story is shaped by the imagery of feudal Britain. The reader knows that Arthur's reign will coordinate the minor petty kingdoms of the area into one nation-state, but the novel depicts that change through time, starting with a disparate collection of various kingdoms, and moving toward the national loyalty to Arthur. He adventures through what will one day become his kingdom.
Knights and castles
Arthur doesn't just get to be a king. He has to train. For a while, he is becoming a knight, learning swordplay and chivalry from Merlin. Then, he has to journey through feudal Britain helping others, and finally, he comes into his kingdom. The imagery that describes his journey is deeply knightly, and he is shown through that quest for holy knighthood to be a person of high quality and character. He is the knight in shining armor.
Magic and witchcraft
The use of magic and witchcraft in the novel forms an important imagery, pointing the reader to the natural forces that underpin daily human life. For instance, Merlin sees Morgause and realizes that she has experimented with evil magic. She can't hide her evil from him, because Merlin is a prophet. He can see things that other people don't, especially in the realm of magic. He is a magician (perhaps the most archetypal magician of all English literature), and he sees the world through this imagery.
Church and the will of God
The will of God is depicted through the story in a similar fashion to its Arthurian predecessors. For example, the imagery of the church and the chapel are central to Arthur's call and coronation. Merlin often claims to be the agent of God's will, especially as the custodian of the king and the king's sword. He excludes himself from the equation by saying, "God himself gave Arthur the sword," but technically, Merlin did. Merlin's character is formed around the assumption that fate is in charge of the plot. He is merely an archetypal arbiter of that fate.
