This is arguably the weakest of the Narnia books. (I don’t mean the worst, merely the weakest.) Lewis didn’t seem have any new ideas yet for what to do with Narnia, so he essentially reworked the first book. The children come to save Narnia from the grip of a foreign power that is hostile to “real Narnia,” “old Narnia.” There is a gearing up for battle and then the children separate, the boys staying to show their manliness in combat, the girls riding with Aslan on a joyous Bacchanalia to awaken allies.
This is not, from the point of view of a child devotee, neccessarily a bad thing. In the wise words of my friend Susie (regarding a different series), “it’s like getting to read your favorite book over again for the first time.”
But in virtue of it being a re-tread we do spend most of the book in Narnia, and for the last time. Remarkably, the Narnia books are not primarily set in Narnia. They take place mostly beyond its borders: to the East or North or South or West; before it begins or after it ends. Narnia is the anchor, the thing we are always trying to get back to, but it is not the main event.
C.S. Lewis wrote a book of essays called Surprised by Joy, about his conversion to Christianity, but it turns out his definition of “joy” was a terribly warped and sad one. According to Miller’s invaluable book, he was talking about the sudden, piercing longing that can strike a person, when a sound or sight conjours up a vision of something you desperately want and can never have. For some reason, for Lewis, this was “joy.” And this is why we can’t spend prolonged time in Narnia. To do so would be to replace longing with ordinary happiness.