From a quality point of view, this series of books is excellent with laminated pages and solid yet my son is far from a tender with its business. The quality is very good explanations, the designs are simple but well illustrate the text. For the specific case of "The strong castles," we are told the Middle Ages, building castles with masons, carpenters, blacksmiths. We also have different parts of the castle, it quon done it. The knights, peasants, attacks the castle, everything is there! It's very rewarding for children from 3 years and much more!