BBC documentary here looking into an idea that C.S.Lewis used ideas from mediaeval cosmology in each of his Narnia Chronicles. Given that Lewis was a professor of ancient and mediaeval literature, and had a fascination with mythology and the planets, this would seem to me to fit. It comes out clearest in his cosmic trilogy, not only in Ransom's visits to other planets, but in the characteristics of those planets (our 'Mars' and 'Venus'), and in the last of the trilogy, in the gods descending to earth like Plato's ideals taking shape.
The theory explains why Lewis breaks the laws of a fantasy world by inserting Father Christmas into The Lion, the Witch and the Wardrobe, for example. Knowing the Chronicles practically by heart (I've a wonderful mother who read them to us on Sunday afternoons, and later I recall reading Price Caspian 6-8 times in one year), I find it a compelling theory.
What I don't like is the hype of calling this a code, to unlock Narnia. It isn't a code, showing a deeper meaning. It merely helps us see Lewis' thought behind some of the ideas he put in the stories, to appreciate his thought rather than dismissing it as random. As such, it's wonderful to appreciate Lewis' brilliance more.
Why not delve into Lewis' essays on literature, or other works? I do esteem his last novel, 'Til We Have Faces, as the best fiction he wrote. It masterfully weaves together his love for classics, for myth, for apologetics, and his incredible writing skill - simply marvellous! And beautiful. Lewis himself said it was one of his favourite works.