In the Middle Ages, what was a “castle” mainly used for?

The story behind the answer

Castles were primarily military strongholds built to protect people and control territory in a violent age. Their design focused on withstanding sieges: thick curtain walls, elevated keeps, battlements, arrow slits, murder holes, drawbridges, moats, and heavily fortified gatehouses all created layers of obstacles and lethal “killing zones” for attackers. Positioned on hills or near key routes, they enabled surveillance, rapid mustering, and shelter for locals during raids.

Over time, timber motte-and-bailey forts (common after the Norman Conquest) gave way to stone structures that resisted fire and battering. By the 12th–13th centuries, concentric layouts—multiple rings of walls—reflected lessons from Crusader fortifications. While many castles housed lords, courts, and stores, these roles were secondary to their function in siege warfare, garrisoning troops, and projecting control over roads, bridges, and borders.