There are two excerpts from Lewis' writing about allegory and symbolism on this page.

Discussion of allegory and symbolism in Allegory of Love, Chapter II

. . . This fundamental equivalence between the immaterial and the material may be used by the mind in two ways . . . . On the one hand you can start with an immaterial fact, such as the passions which you actually experience, and can then invent visibilia to express them. If you are hesitating between an angry retort and a soft answer, you can express your state of mind by inventing a person called Ira with a torch and letting her contend with another invented person called Patientia. This is allegory . . . . But there is another way of using the equivalence, which is almost the opposite of allegory, and which I would call sacramentalism or symbolism. If our passions, being immaterial, can be copied by material inventions, then it is possible that our material world in its turn is the copy of an invisible world. As the god Amor and his figurative garden are to the actual passions of men, so perhaps we ourselves and our 'real' world are to something else. The attempt to read that something else through its sensible imitations, to see the archtype in the copy, is what I mean by symbolism or sacramentalism. It is, in fine, 'the philosophy of Hermes that this visible world is but a picture of the invisible, wherein, as in a portrait, things are not truly but in equivocal shapes, as they counterfeit some real substance in that invisible fabrick'. The difference between the two can hardly be exaggerated. The allegorist leaves the given--his own passions--to talk of that which is confessedly less real, which is fiction. The symbolist leaves the given to find that which is more real. To put the difference in another way, for the symbolist it is we who are the allegory. We are the 'frigid personifications' ; the heavens above us are the 'shadowy abstraction'' the world which we mistake for reality is the flat outline of that which elsewhere veritably is in all the round of its unimaginable dimensions. . . .

. . . Symbolism comes to us from Greece. It makes its first effective appearance in European thought with the dialogues of Plato. The Sun is the copy of the Good. Time is the moving image of eternity. All visible things exist just in so far as they succeed in imitating the Forms.

Pages 45-46

A picture of Meekness (Allegory)
A statue of Truth (Allegory)

From "a letter to a lady," dated 29 December 1958

By an allegory I mean a composition (whether pictorial or literary) in wh. immaterial realities are represented by feigned physical objects; e.g. a pictured Cupid allegorically represents erotic love (which in reality is an experience, not an object occupying a given area of space) or, in Bunyan, a giant represents Despair.
If Aslan represented the immaterial Deity in the same way in which Giant Despair represnets Despair, he would be an allegorical figure. In reality however he is an invention giving an imaginary answer to the question, 'What might Christ become like, if there really were a world like Narnia and He chose to be incarnate and die and rise again in that world as He actually has done in ours?' This is not allegory at all. So in Perelandra. This also works out a supposition. ('Suppose, even now, in some other planet there were a first couple undergoing the same that Adam and Eve underwent here, but successfully').
Allegory and such supposals differ because they mix the real and the unreal in different ways. Bunyan's picture of Giant Despair does not start from supposal at all. It is not a supposition but a fact that despair can capture and imprison a human soul. What is unreal (fictional) is the giant, the castle, and the dungeon. The Incarnation of Christ in another world is mere supposal; but granted the suppposition, He would really have been a physical object in that world as He was in Palestine and His death on the Stone Table would have been a physical event no less than his death on Calvary.

In Letters of C.S. Lewis Ed. Warren Lewis, page 283