Faith under pressure
The book keeps testing whether characters will trust what they can't fully see or prove. Lucy believes in Aslan when no one else does. Caspian believes in Old Narnia based on stories alone. Lewis shows that acting on that belief, even when it's costly, is what moves the story forward.
Legitimate authority versus usurped power
Miraz holds the throne but has no right to it. Caspian is the true heir but has to prove he deserves to rule. The book draws a hard line between power grabbed by force and authority earned through character and lineage.
The danger of desperation
Nikabrik's arc shows what happens when a good cause starts losing. He begins as a skeptic and ends up trying to resurrect the White Witch. Lewis uses him to show that fear of failure can push people toward the very evil they set out to fight.
Memory and loss
The Pevensies return to a Narnia that barely remembers them. The talking animals are in hiding. Cair Paravel is rubble. The book is full of things that were once good and are now gone or buried. That loss drives both the grief and the motivation to restore what was lost.
Growing up and leaving behind
Peter and Susan are told at the end that they won't come back. Their time in Narnia is finished. Lewis frames growing up as a real change that closes some doors permanently. The book doesn't pretend that's painless.