World War II-Era England; Narnia
Most of The Lion, the Witch, and the Wardrobe takes place in the fantastic land of Narnia, which Lucy and her siblings reach through—you got it—a magical wardrobe.
Narnia is everything we've come to expect from a fantasy novel, largely because this book (along with J.R.R. Tolkien's Lord of the Rings trilogy) helped to set the standard for what we think of as a typical fantasy world. It's a vaguely medieval place in which people live close to the land, fight using bows and arrows and swords, and are ruled by kings and queens who live in palaces. Mythical creatures populate it – not only the stock characters of today's fantasy world like centaurs and dwarves, but more Greco-Roman-feeling characters too, like minotaurs and dryads.
Interestingly, Narnia has no human population, but prophecy states that it needs to be ruled by four human beings, two male and two female—two "sons of Adam" and two "daughters of Eve," as the characters are constantly reminding us.
Perhaps more interesting than the fantasy land of Narnia is the world that the Pevensie children escape from for their adventure. At the beginning of the book, Peter, Susan, Edmund, and Lucy have just been evacuated from London to live in a house in the country. The book doesn't use the name "World War II," but that's exactly what they've been sent away from the city to avoid.
Thousands of English children were sent away from London because of bombings and air-raids during the 1940s, often to live with strangers who volunteered to take them in. So these four children escape from one cataclysmic, world-changing battle only to find themselves in the midst of another.
Maybe Narnia isn't that much different from the "real" world after all...though Good and Evil may be more obvious there.