They have flags because a flag can show one of two things : The lords coat of arms, and so which noble owns the castle the colours of the lords mater for an example a kings coat of arms or a country's flag , showing where its allegiance lies.
Castles have flag because they depict the country that has hold over it.
Castles display flags primarily for identification, status, and allegiance, showing ownership and loyalty. They also serve as deterrents against attacks and represent the traditions of the feudal system. Flags played a crucial role in the communication and recognition during the medieval era.
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