In The Lion, the Witch, and the Wardrobe, several elements, such as the lamp post, the wardrobe, the White Witch, etc. are included in the plot without any explanation or backstory. Several books later in The Magician’s Nephew, C.S. Lewis provides explanations for how these things came about. Did he have these explanations in mind when he wrote the first book or did he make them up afterwards?
Based on the origins of the lamp post alone, I would almost think that he had the information in mind already. It seems kind of out of place alone in the “Lantern Waste” in a magical world like Narnia?