Lucy and her brothers and sister enter Narnia through a magic wardrobe – a piece of furniture intended to be used as a closet in rooms that don't have built-in closets. (If you're interested in where this particular wardrobe came from and why it's magic, we suggest you read The Magician's Nephew.) Lucy is drawn into the wardrobe because she wants to feel the fur coats that are hanging in it. Instead of having a normal back, though, the wardrobe opens out into the woods in the west of Narnia:
Next moment she found that what was rubbing against her face and hands was no longer soft fur but something hard and rough and prickly. "Why, it is just like branches of trees!" exclaimed Lucy. And then she saw that there was a light ahead of her; not a few inches away where the back of the wardrobe ought to have been, but a long way off. Something cold and soft was falling on her. A moment later she found that she was standing in the middle of a wood at night-time with snow under her feet and snowflakes falling through the air. (1.24)
We notice that, instead of going out into the world of Narnia, Lucy goes further and further inward. After all, she is only exploring the Professor's house because it's raining outside and so she can't go outside to explore the country. In the depths of the house she finds a spare room, and in the spare room she finds the wardrobe, and in the wardrobe she finds all of Narnia. OK, we know, if you read the other books, it becomes obvious that the wardrobe is a door and that Narnia isn't actually all contained in the wardrobe, but symbolically it kind of feels that way, in this book, at least. We might also think of Narnia as "inward" in a more abstract sense – to find her adventure and her destiny, Lucy, along with her siblings, goes deeper and deeper into herself.
The other interesting thing about the wardrobe is that it doesn't work all the time. After Lucy's first trip to Narnia, she tries to show the others, only to discover that now the wardrobe has a normal wooden back. The next time she tries, it is once again a magical gateway, and this time Edmund gets through too – but again it switches back to cupboard form when Peter and Susan come in. Peter and Susan feel that, if something is real, it must be real all the time, but the Professor suggests reality might be more complicated than that.
Oh, one last thing about the wardrobe – C.S. Lewis wasn't the first person to write a fantasy story for kids in which the protagonists get to the magical world through a wardrobe. To find out who was, check out the "Trivia" section!