Door in the Wall by Marguerite de Angeli

Robin has grown up the son of a nobleman. He knows he must serve the king by becoming a knight. But Robin's destiny changed suddenly when he falls ill and loses the use of his legs.

A monk named Brother Luke rescues Robin and takes him to the hospice of St. Mark's. There Robin learns woodcarving and --- much harder --- strength and patience. When danger threatens the great castle of Lindsay, Robin discovers there are more ways to serve a king than riding into battle. As Brother Luke says, "Thou hast only to follow the wall long enough and there will be a door in it."