A castle becomes a fort when it serves the state

When did castles become military forts?

Broadly, when a defended personal or royal residence became a state-owned national military asset, although historical terminology is not perfectly consistent.

Castle and fort are overlapping terms. A castle begins as the defended residence and personal asset of a monarch or noble. Once government ownership and national military use replace that personal role, fort becomes the more useful term. The terminology remains approximate, since historical usage does not always follow that rule.