A castle is a fortified residence for a medieval noble. Castles come in all shapes and sizes, but knowing a few general terms will help you understand them. The Keep (or Donjon): A high, strong stone tower in the center of the castle complex that was the lord's home and refuge of last resort.

A castle is defined architecturally and historically as a fortified private residence of a lord or noble, typically built during the Middle Ages. Unlike a "fortress," which is a purely military structure designed for defense and housing troops, or a "palace," which is a grand unfortified residence designed for display and luxury, a castle must combine both defensive features and domestic accommodation. Key structural elements often include curtain walls, moats, keeps (donjons), and battlements. The evolution of the castle saw it transition from simple "motte-and-bailey" wooden structures to massive stone complexes designed to withstand sieges and the advent of gunpowder. In a modern 2026 context, many structures called "castles" (like Neuschwanstein in Germany) are actually "castle-style" palaces or châteaus built for aesthetics rather than military function, but the true historical definition remains rooted in the dual-purpose of being a fortified home for the ruling elite.