Informative, Straightforward, Simple
While in some ways Lewis keeps himself on the straight and narrow because he's writing for children—no big words, no long chapters, no overly complex ideas—he includes some big thematic concerns that he conveys in clear and careful prose. Consider, for instance, Aslan's explanation of Caspian's death and resurrection:
He has died. Most people have, you know. Even I have. There are very few who haven't. (16.240)
Very spare, clean, no fuss. In a moment when a rather complicated idea is being expressed, the writing is stripped of frills, making sure readers can access what's being said without anything getting in their way. Lewis approaches the descriptions of the most fantastical elements of Narnia in the same way. For instance:
And what a bridge, too! It was a huge, single arch that spanned the gorge from cliff-top to cliff-top; and the crown of that arch was as high above the cliff-tops as the dome of St. Paul's is above the street. (6.84)
Note that he is careful to use readily accessible images (we may not all know what St. Paul's looks like, but Lewis' British readers at the time would have), short sentences, and simple vocabulary to convey sheer awe at this construction. That's pretty much how he rolls throughout the whole book—giving us the info we need at every turn, and grounding his fantasy world in reality whenever possible to help us get our bearings.