Setting is a lot like geology. Wait, don't go! We're being serious here.
See, geology studies how forces like plate tectonics, climate change, and fluxes in the Earth's core have shaped our world. When studying setting, you'll want to look for how forces like themes, symbols, and philosophy have molded the place where the story happens. Or how the story molds those forces—since cause and effect goes both ways.
The forests and rivers of Lewis's Narnia provide an excellent place for budding settingologists to start their careers.
On the Map
The Pevensies last visited Narnia a year ago by England's standards, but a whopping thousand-some-odd years have passed for Narnia. In that time, the mystic land they once thought of as their kingdom has changed drastically.
Thanks to Dr. Cornelius's history lesson, the readers learn what happened along with the young Prince Caspian. As he tells it:
"All you have heard about Old Narnia is true. It is not the land of Men. It is the country of Aslan, the country of the Walking Trees and Visible Naiads, of Fauns and Satyrs, of Dwarfs and Giants, of the gods and the Centaurs, of Talking Beasts. It was against these that the first Caspian fought. It is you Telmarines who silenced the beasts and the trees and the fountains, and who killed and drove away the Dwarfs and Fauns, and are now trying to cover up even the memory of them." (4.53)
Narnia is now under the rule of a race of men called the Telmarines, and the magical creatures of yore have become creatures of lore. Now, two major cultures live in Narnia: the Old Narnians and the New Narnians. The New Narnians are the Telmarines; the Old Narnians are the dwarfs, fauns, centaurs, and so on.
These social changes are reflected in the landscape of Narnia. If your edition has a map (or if you have a finger to click on this one), you'll notice that Miraz's castle and other New Narnian havens tend to hang out in the northlands. As we later learn, Telmarines fear both forests and seas because their history has it that "Aslan comes from over the sea" and "they have quarreled with the trees" (4.73). So the Old Narnians congregate in the southlands where the forests are thick and the sea stands beside them. Also in the southlands are such historical sites as Cair Paravel, Aslan's How, and Beruna Ford.
Digging Deeper into that Bedrock
As promised, let's take a look at how themes mold the setting of Narnia and vice versa. Obviously, there are plenty of themes, and you'll hunt for different clues in the setting depending on which one you choose to focus on. Here, we'll take a stab at how the setting reflects the theme of "Memory and the Past," and then you can do the same for whichever theme you want.
Sound good? Excellent.
If you check out our discussion on "Memory and the Past," you'll see that Prince Caspian is all about returning Narnia to a past era. The Old Narnians see the reign of Peter, Susan, Edmund, and Lucy as the good old days, so their goal is to bring back that golden era with Prince Caspian as the king.
For us, the best representation of this in the setting is Beruna. When Aslan invades the town, the first thing he does is have Bacchus destroy the bridge. With the bridge down and the rivergod free, the Bridge of Beruna becomes the Ford of Beruna once again. Lucy and Susan cheer "'Hurrah! It's the Ford of Beruna again now,'" clearly excited to see the place return to how they remembered it (14.53). And let's not forget that Peter and Edmund won a "glorious victory" there, further connecting this ford with an idyllic version of the past (10.12).
As Aslan moves through Telmarine territory, most of the Telmarines flee, and the animals are freed to join Aslan's party. In buildings, "[t]he walls [become] a mass of shimmering green, and leafy branches [arch] overhead where the ceiling[s] had been" (14.59). In other words, Aslan is transforming the human villages to resemble the more forested, natural setting of ye golden days.
Ta-da! Just like that, you can see how the theme of "Memory and the Past" directly affects the setting. In this case, like geology, the theme literally shapes the landscape.
Now, the relationship between theme and setting might be a tad subtler with other themes, but don't let that intimidate you. Go forth. Your promising career as a settingologist awaits you.