What are castles and knights?

"Castles may conjure up a romantic fairy tale world; however, in real life, during the Middle Ages, castles were fortresses, providing shelter and protection for the lord as well as for the peasants who lived on his land. For an army, a lord depended on young soldiers in armor called knights who spent years at the castle learning the skills of warfare. [This book explores the world of castles and knights]"--Provided by publisher.