mansion vs castle

mansion and castle both are nouns.

Word	Noun	Adjective	Verb	Adverb
mansion	Yes	No	No	No
castle	Yes	No	Yes	No

As nouns, castle is a hyponym of mansion; that is, castle is a word with a more specific, narrower meaning than mansion:

Other hyponyms of mansion include manor, manor house, palace, stately home, mcmansion.

mansion (noun)	castle (noun)
a large and imposing house	interchanging the positions of the king and a rook
(astrology) one of 12 equal areas into which the zodiac is divided	a large building formerly occupied by a ruler and fortified against attack
	(chess) the piece that can move any number of unoccupied squares in a direction parallel to the sides of the chessboard
	a large and stately mansion

mansion (verb)	castle (verb)
	move the king two squares toward a rook and in the same move the rook to the square next past the king