What are the advantages of castles?

Castles were forts. They had the advantage of security, and for this purpose they were strongly built so a limited garrison could defend against a much larger force. They had rooms for food storage and often had roofs to catch water for storage in their cisterns. Their walls were thick and built of stone. Typically, they were built on mounds, hills, or islands, where siege equipment would be difficult to bring into play.

A very few castles were built so a lord and his family could be tolerably comfortable there.