Wardrobe

The wardrobe was a magical portal that linked Earth through Professor Kirke's House to the world of Narnia.

History
The wardrobe was created when Digory Kirke, who later becomes the Professor, had his own adventure in Narnia. Aslan sent him to get a silver apple from a magic tree and bring it back to him. When he did, the apple was planted right there in the ground, and a beautiful tree grew, with silver bark. Aslan later gave one of its own apples to Digory to take home to his sick mother. When Digory returned to London and gave her the apple, she ate it and soon recovered from her long sickness. On impulse, Digory planted the apple core in his back yard, and it soon grew into a tree as well - a daughter tree to the one that had grown in Narnia. It is noted in the book that sometimes the apple tree would creak and move as if being blown in the wind - when there wasn't any, as if it could feel the winds that blew it's mother tree.

Portal to Narnia
Eventually, the tree was blown down in a storm, and the much older Digory, unable to bear the prospect of the magical tree being simply chopped up for firewood, had the wood from the tree made into a beautiful wardrobe. This was placed in an empty back room in his large country home, where many years later Lucy Pevensie discovered it during a game of hide and seek. Hiding inside it, she felt tree branches and snow, and looked up to find herself in Narnia, in Lantern Waste, near the Lamp-post. The portal seemed not to be open all the time, because when Lucy returned and tried to show the other children Narnia, they found only an ordinary wardrobe, with a solid back. The wardrobe portal was used twice more, first by Lucy and Edmund who had separate adventures, then  finally all four children entered together. They saved Narnia from the White Witch and ruled Narnia for 15 years during the Golden Age. They came back through the wardrobe the same way, and told the Professor about all of their adventures. He said they would return to Narnia someday, but suggested they not try to get into Narnia by that route again.

Inspiration
Author C. S. Lewis had an English country home of his own, and he took in children who were fleeing London because of the German air-raids much the same as the Professor did. He also had a magnificent wardrobe in an upstairs room, said to have been of particular interest to one little girl, and to have inspired the wardrobe for which Lewis is now known.