Final answer:
Castles built during the early Middle Ages were constructed mainly out of stone, and served as places of refuge for both nobles and peasants during times of war. They were difficult to destroy by fire, which is why many of them still survive.
Step-by-step explanation:
During the early Middle Ages, castles were primarily constructed out of stone. These castles were not built by peasants to house their families, but rather served as places of refuge for both nobles and peasants during times of war. Castles were designed to be difficult to destroy by fire, which is why many of them still survive to this day.