Tolkien, I recall, had reservations about Lewis's blending of inconsistent sources and traditions in the Narnia stories: Roman fauns and Greek centaurs against Nordic wolves and a White Witch, for instance. But I wonder what he made of T. H. White's The Sword in the Stone, an extraordinary melange of elements: sentimental medievalism blended with precise historical detail and conscious anachronism, children's story and searching political allegory, legend and fantasy, and comedy with a looming thread of adult tragedy.
Among those concerned with writing modern fantasy, much is made of world-building, which means the coherence and integration of the world, and the application of a realist logic to unreal categories. But as M. John Harrison has observed, fantasies are extended metaphors, idealogical and not physical landscapes, and they serve to generate interpretations, not factual consistency. In this respect, The Sword in the Stone is a novel of education, but its lessons are about encountering and integrating, without necessarily reconciling, a multitude of viewpoints.
Merlin, living backwards in time, has already seen the tragic future and the even more baffling modernity which occasionally intervenes in the text, but he can only prepare Wart for what he will discover and ultimately do as the Once and Future King. He can guide but not resolve. Wart's lessons, through Merlin's magical transformations, are often about the burden of power, and his encounters, such as with the mordant pike or the mad, militaristic hawks, highlight the dangers of tyranny and the moral price of authority. If the fantasy England of the narrative is but a nostalgic rendering, this serves to highlight that it is also lost in the past as the whole golden age of Arthurian legend is lost, that these soft edges are just a comforting illusion, but one we can't give up quite yet. This is what Merlin sees and Wart cannot, and the discrepency lends the comedy unusual poignancy and insight.