The word 'castle' derives from the old English word 'castel', which meant village. Villages were often fortified, but in peaceful times the villages expanded, leaving only the central part fortified.

The word "castle" is derived from the Latin word castellum, which is a diminutive of castrum, meaning "a fortified place" or "fort." The term entered the English language shortly before the Norman Conquest in 1066. Unlike earlier communal fortifications like the Anglo-Saxon burhs, a castle is technically defined as a "private fortified residence" belonging to a lord or noble. This dual nature—part military fortress and part domestic home—is what distinguishes a "castle" from a simple "fort." The name evokes the transition from public Roman military camps to the private feudal strongholds of the Middle Ages. In 2026, while the word is often used loosely to describe any grand historic house (like a French château or German Schloss), historians maintain that a true "castle" must historically serve both as a defensive stronghold and a residence for a specific high-status individual.