Tolkien hated it
C.S. Lewis and J.R.R. Tolkien were close friends who read each other work in progress for years, and Tolkien thought NARNIA was a mess. His objection was not religious — he was a devout Catholic and it was partly his arguing that brought Lewis back to Christianity — but a matter of craft.
Tolkien spent decades constructing Middle-earth so that its languages, maps and histories held together. Lewis put a FAUN from Roman myth, Father Christmas, a talking BEAVER, a DWARF, a CENTAUR and a LAMPPOST from a Victorian street into the same wood and did not think the mixture needed explaining. To Tolkien this was carelessness. He also disliked the speed: Lewis wrote the first book in a few months, and the whole seven in about seven years, while Tolkien had been revising one story since the First World War.
The order problem
The books were published between 1950 and 1956 starting with The Lion, the WITCH and the WARDROBE. The Magician's Nephew, published sixth, is a prequel explaining where NARNIA came from, how the LAMPPOST got into an empty wood, and why a WARDROBE built from a particular tree opens onto another world.
Modern boxed sets are usually numbered in internal chronological order, which puts the prequel first. This annoys a lot of readers, because the wardrobe scene works by surprise and the prequel spoils the surprise. Lewis was asked directly by an American boy in 1957 and replied that he rather preferred reading them in chronological order himself, though he did not think it mattered much — a mild answer that both camps have been quoting at each other ever since.
It started as a picture, not a plan
Lewis said the whole thing began with an image he had carried since he was about sixteen: a FAUN in a SNOW-covered wood, carrying an umbrella and parcels, standing by a LAMPPOST. He had no idea what it meant and did nothing with it for twenty-five years. At about forty he decided to try building a story around it, and ASLAN came bounding in, as he put it, without being invited.
That is why the opening chapter is the best-known part. It is the bit he actually saw. The rest was assembled outwards from it — the evacuee children sent out of London during the war, which happened in Lewis's own house, the White WITCH's endless winter, Turkish DELIGHT as the exact sweet a rationed 1940s child would betray a sibling for, and the CASTLE at Cair Paravel where the four of them end up crowned.
What the grid holds
The four Pevensies — PETER, SUSAN, EDMUND and LUCY — plus ASLAN the LION, Mr TUMNUS the faun, the White WITCH and the MAGIC of a land under permanent snow with no Christmas. LUCY is the one who gets through first and is disbelieved, which is the engine of the whole first book.
Later volumes supply the rest. King CASPIAN and the mouse REEPICHEEP come from Prince Caspian and The Voyage of the Dawn TREADER, in which a ship sails east to the world's end and Reepicheep goes over the edge in a coracle rather than turn back. If the mythical furniture is the appeal, the FAUN and the CENTAUR both walked in from Greek mythology more or less unaltered.