What is the difference between a castle and a mansion?

Answer 1

Size. A castle is larger than a mansion even though some mansions look like castles.

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A castle is essentially a fort. Castles had certain characteristic features, including a central ward, which was a large courtyard in which there were often one or more buildings, and these might include a house for the owner. The ward was surrounded by a curtain wall, which was often just a thick and tall stone wall with a walk on the inside at the top where defenders could position themselves. There would be a gate, defended by a gatehouse, and often a moat.

A mansion is a large and imposing house, and the home of a wealthy person. Medieval manor houses sometimes were fortified, and when they were they looked like castles, but were much smaller and did not have all the characteristic features. Fortified manors often had the moat, but they did not have a central ward, curtain wall, or gatehouse.

Metaphorically, a mansion is sometimes called a castle, but the usage is imprecise.