A castle is a building built in the form of a very large fortified residence, typically featuring watchtowers and castellations. There is often some form of limited access at the base, such as a moat, a drawbridge, or a steep approach. Castles are most often situated on sites which are difficult to access, such as islands, mountainsides, or inaccessible shorelines.

If the parts of a castle are clearly separated from each other at ground level, then it can be a complex with component parts listed as guard towers, churches, and low-rise buildings.

A fortified military structure, which is not built as a permanent aristocratic residence, is classified as a fortress.