The oracle pig
The holy grail of this story's quest is a pig who is very valuable because somehow, by natural pig processes, the pig is able to provide insight about the future. This elevates the pig to a level hitherto unheard of for mere animals; it is the treasured fascination of all the kingdom's most important and powerful forces, leading to an epic and archetypal story of good and evil. The pig seems also to be a symbol for what pigs have historically represented in human mythology; the factor of competence (pigs are a natural symbol for competition because of their tusks and mating practices).
The evil Spiral Castle
There is strong archetypal significance in the idea of evil being contained within the mystical walls of some ancient castle whose roots seem to be in the sky. The spires of the castle seem to drag down some sacred energy from space or the heavens above, like religious authority. Therefore, evil is located within the kinds of order which are easily perpetuated, like systems of authority who go chronically unquestioned or unchallenged. When systems of order don't have to change to adapt to new times, unfortunate dynamics emerge that set about to decompose the stilted system. That is the archetypal significance of the hero's quest into the evil castle.
The road of trials
The team of this novel must prove their worth to the reader and the novel. They endure a season of trials and challenges which expose their strengths and weaknesses. Some of the challenges force the characters into atypical behaviors that represent the growth of their dynamic character, but others show a kind of stunted progress and a refusal to change. When the pig disappears, the road of becoming is shown as the major action of the novel, not the pig. This is hinted at by the Eilonwy's explanation.
The escape of deathly bird
There is an important German folktale that is found in various early German story collections which is famously depicted in Jung's Archetypes of the Collective Unconscious, in which a hero climbs a tower in the sky to challenge tyrannical forces that dwell there, in places of power, and he accidentally frees a bird from that tower. This archetype is also found in the Revelation of St. John when various kinds of evil are unleashed in stages with animal symbolism, like the four horsemen. Basically, the idea is that with enough vantage (because birds are naturally able to see great stretches of land), a person could see great doom and evil in the future, because those elements are always lurking somewhere. By climbing the tower and freeing the bird, the hero learns about evil and its eternal nature. He isn't "fixing" the earth, but rather, the earth and its nature are shaping him.
The completion of adventure
Upon the completion of the adventure, the book's protagonist receives a debrief which basically functions as a coda or epilogue to the action of the book. This is where a voice of reason will explain more specifically how the symbolism of the book was intended, and what it should signify to the reader. This fairytale motif shows the reader that the true nature of the narrative was to create a character through stages of evolution which are symbolized by animal dynamics. That is an archetypal and allegorical interpretation of the story.
