In some ways, my experience reading this book was influenced by the fact that I had just read Adam of the Road. At first glance, both books are very similar. Each involves a young boy's coming of age as he is separated from his parents and goes on a long journey to find them again. Reading the two stories back to back made The Door in the Wall feel a little bit more tedious than it otherwise would have. That said, the two books are different in a variety of ways. For one thing, The Door in the Wall focuses on a darker side of the Middle Ages. In Adam of the Road, the main character is a minstrel and there is a sense of fun and adventure in everything that happens. The Door in the Wall deals with plague, disability, hardship, and battle, and there are several situations that are truly a matter of life-and-death. The Door in the Wall also conveys a very specific message about making the best of one's situation and looking for opportunities to overcome difficulties. While it is not necessarily a preachy book, it has a much clearer moral than Adam of the Road.
Though this book is short, I would not say that it is an easy read. Upper elementary students are probably the best audience, though it might work as a read-aloud with second or third graders interested in the time period. From an educational standpoint, the story teaches a lot about the role of religion during the Middle Ages, and it gives kids an opportunity to witness day-to-day living in a monastery, on the road, and in a castle. For a short book, it does manage to pack in a lot of great details which will help contextualize lessons on this time period. Robin is also a likable and humble hero, which makes him easy to relate to even when he is accomplishing feats most able-bodied contemporary children would never imagine themselves attempting.
I am thankful, that I didn't have to live in medieval Europe as I don't think I would have lasted very long! The Door in the Wall is an excellent book, but I recommend reading it in isolation rather than in combination with a lot of other medieval stories as I have just done.