When the Pevensie children pop into Narnia, the first building they come across is a ruin. Not exactly a royal welcoming party, is it? As they walk through the rundown mess, they can barely make out that the place was once a building:
While they were talking, they had crossed the courtyard and gone through the other doorway into what had once been the hall. This was now very like the courtyard, for the roof had long since disappeared and it was merely another space of grass and daisies, except that it was shorter and narrower and the walls were higher. (2.7)
As they explore further, Peter comes to the conclusion that these are "'the ruins of Cair Paravel itself'" (2.27). Remember: Cair Paravel was the castle the four ruled in as the high kings and queens of Narnia back in the good old days (i.e. The Lion, the Witch, and the Wardrobe).
The ruins give us the first clues about Narnia's current predicament in the form of a symbol. That is, the Telmarines have destroyed the past glories of Narnia and left them to be forgotten in the woods.
But the ruins don't just hint at the current situation. They also clue us into an important theme: "Memory and the Past." Before we even learn of the ongoing war, the novel is prepping us to understand that the past of Narnia has been rotting and that the rebuilding of that past will be the focus of Pevensie children's quest in Narnia.
Huh, for a busted down building, these ruins certainly do a lot of literary leg work.