From a netfriend:

For Lewis, “allegory” was limited to writing where abstract concepts were represented by material forms. So a book like “The Fairy Queen” where Sir Jurisprudence rescues Lady Justice from the Dragon Tort; or “The Pilgrims Progress” where Christian is imprisoned in a Tower called “Doubt” by a Giant called “Despair” but frees himself with a key called “Faith” are Allegorical. A book like “Animal Farm” where the Major, Napoleon and Snowball have fairly obvious parallels in Karl Marx, Stalin and Trotsky is not Allegorical. (Incidentally, the characters in “Animal Farm” are, I think, “themselves” as well as being analogies for historical figures. Come to that, I think that some of the characters in “Pilgrims Progress” have a narrative realism as well as being symbols, particularly in the second half.)

Yes, this is a technical, pedantic definition of Allegory, as you might expect from the author of “The Alleogry of Love”. In ordinary language, you could say “Lord of the Flies” is an allegory of the Fall of Man, and everyone would know what you meant.

On this definition, of course Narnia isn’t an allegory. (And even it if it were, Edmund would not be Judas. Edmund doesn’t betray Aslan. Edmund agrees to serve the Witch, thinking that that will make him King of Narnia, and ends up in the Witch’s dungeon. Aslan dies to free him. If Edmund is anyone, he is Adam, or “The Human Race.”)

But in ordinary language, when someone says “Are the Narnia stories allegorical?” they mean “Do they have a Christian point behind them”. To which the answer is “Duh! Of course they do.”