Occasionally the Narnian books will be casually referred to as "allegories." I don't know if anyone reading this posting calls them "allegories." If we take the historically normative use of "allegory," I don't think the Narnian books qualify. The Pilgrim's Progress is a good book and a good example of an allegory that ought to be familiar to everyone. In it a character named Christian runs into trouble in a town called Vanity Fair, and during his journey he meets characters with names such as Watchful, Hypocrisy, Worldly Wiseman, Giant Despair, & so on. Persons, things, places are emblems of spiritual states. Conversely, certain spiritual states are like these emblems.
In the Narnian books, you have children such as Peter, Edmund, Jill, &c. They're not allegories but kids. You have Aslan, who is not an allegory of the Second Person of the Holy Trinity; He is, as readers may eventually realize,* the Second Person as He might be supposed to be incarnated in a world in which animals possess reason, can speak, make moral choices, etc. The White Witch isn't an allegory of evil, she's an evil person. Cair Paravel is a castle, not an allegory of spiritual strength or whatever. Sometimes when people object to the "allegory" of the Narnian books, what they probably are really objecting to is the Christian belief reflected in the books. For many non-Christian readers this isn't a problem, just as for many Christians the Taoism in Ursula Le Guin's first Earthsea books is not a problem.
*Lewis doesn't make it hard for the reader to discern this, but nor does he force the matter on the reader. He wants the stories to be enjoyed by the reader whether the reader unpacks these significances or not, to whatever degree is appropriate. I don't think he wanted busybodies to point out these matters to readers, sort of taking charge of the readers and the books.