From the very beginning, the children of Adam are central to the story of Narnia. Like tributaries from a great river, there are many other worlds linked to Adam’s world by portals mysterious and unknown. One of these worlds is Narnia.
In the history of Adam's sons, the central event is the incarnation of the Son of God in Jesus Christ. Even within Narnia itself, the story of Christ is understood to be much bigger than the story of Aslan.
The narrative-world of Narnia is sometimes called an “allegory.” C.S. Lewis hated that. It was not an allegory.
Aslan was not a symbol of Jesus, Aslan was the one person of the Divine Son as manifest in another nature existing alongside our own. He makes clear to Eustace at the end of the Silver Chair (and in an implication to Peter and Susan at the end of Prince Caspian) that he has been brought to Narnia only so that he might know the one called Aslan much better by the name he bears in our world — obviously Jesus. This is not allegory, anymore than Maleldil is a symbol or allegorization of Jesus in the Space Trilogy.
But it seems to me that he did not conceive of the world of Chronicles of Narnia as being merely constituted by a host of separate natures which linked together without any overarching structure (the idea of distinct natures fashioned by one Creator held Lewis’ imagination and was discussed in both Miracles and Problem of Pain), but instead sees the created order as being constituted by a core world — our own — joined by various threads, historical, temporal, and personal, with countless other worlds. We are only given a look into two other than ours, though it is clear in Magician’s Nephew that there are many others.
In Charn, Jadis (the White Witch) knows of the race of Adam and mentions our first parents by name. This suggests that there is a centrality to our world that requires it be known to some degree in her own, while obviously not being true in reverse. Some might object that Charn is called an older world than ours, but remember that time proceeds in various relative speeds (Lewis’ reflections on God and time permeate his whole published work, including Mere Christianity), so that it might be that Charn was created after our world and in relation to it but extended for tens of thousands of years even as our own was much younger (yes, I said it).
We are given a clearer vision, obviously, of Narnia. Narnia lasts about two-thousand years of internal time, but from the very beginning the children of Adam are central to its story. Talking Beasts are not conceived of as being that world’s equivalent to man, not simply a beast-shaped human. Rather, Lewis conceives of them as really animals but exalted and lifted up in relation to Adam’s family. In the Last Battle, at the final judgment, the wicked beasts end up becoming dumb beasts again, but this is not true for wicked Narnian children of Adam. Likewise, it is clear that royal dominion belongs to and only to the flesh and bone of Adam. King Frank is taken from our world. The Telmarines, likewise, passed from our world to the world in Narnia by an unexpected portal. The Calormenes came the same way, I think, though I don’t remember it being specified. But like in Charn, the Narnians are aware of our world, but men of our world are not aware of theirs.
Our world also seems to be quite a bit larger than the Narnian world. In Last Battle, we learn that the stars are real, living, conscious beings that are very close to the flat world which is the landscape — this is contrasted with the round, supermassive cosmos of our own world.
So it seems to me right to think of Lewis’ conception of his story-universe as having a central backbone constituted by the history of Adam’s sons and having as its central event the incarnation of the Son of God in Jesus Christ. But like tributaries from a great river, there are many other worlds linked to Adam’s world by portals mysterious and unknown. And to each of them (by implication) a few children of Adam and Eve are carried to continue and multiply outwards Adam’s story in the tributary-worlds, God bringing these various chapters to their various conclusions with the end of bringing their glorified states (the “true Narnia” of the last chapter) into a single creational unity with the the core world — our own — when all is said and done and the Lord is all in all.