"The Princes and the Goblin" is a prime example of his genius. While not a truly masterful wordsmith, his grammar and paragraph structure were moderate, MacDonald can build worlds that suck people in and keep them.
On the surface it is a simple adventure story of a miner boy who rescues the kingdom's princess from the goblins that live below the mines. Far deeper threads of thought run through the story, twining with the tale above much as the miners in the story do. They lead the reader to ask questions and seek answers. And that is quite a lot for a princess and a miner boy.