Knight's Castle, by Edward Eager, Harcourt Brace, 1956.
Knight's Castle is not my favorite book Edward Eager's magical series. Even so, it is a fun read.
The original children from Half Magic have grown up, and two of them have had children. The story of Knight's Castle was probably ever bit as good as the earlier Half Magic, but I just didn't warm up to the children in this book as quickly as I did to the ones in the first book.