For questions on castles, a type of fortified structures common in Medieval Europe, and analogous fortifications found elsewhere in the world.

The castle is distinguished from other types of fortifications in that typically, it was, or could be used as, a residence for the nobility. They emerged in Europe during the High Middle Ages as the power base of local nobles, offering protection against enemies while establishing control of surrounding regions.

A similar structure found in Japan is the shiro, which like the European castle is a fortified residence for the nobility. Japanese castles developed independently of the European concept and have significant architectural and organisational differences.

Many buildings which are not technically castles contains the word "castle" in their names, and are often mistakenly referred to as such.