Contextual explanation: Fortified residence and defensive structure. 
Castle is best understood as a type of structure—specifically, a large, fortified building or complex designed for both defence and residence, historically associated with royalty or nobility. Its defining features include thick, high walls, towers, and strategic placement for protection and control.

Themes: Security and authority. 
The concept of a castle is closely tied to the theme of security, as castles were constructed to provide protection against external threats. They also symbolise authority, serving as seats of power and governance for those who inhabited them.

Nuance: Defensive architecture for elite residence. 
What distinguishes a castle within its category is its dual function: not only as a defensive stronghold but also as a prestigious residence for the elite. This combination of military and domestic roles, along with its architectural complexity, sets castles apart from other types of structures.