castle vs mansion

castle and mansion both are nouns.

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

As nouns, mansion is a hypernym of castle; that is, mansion is a word with a broader meaning than castle:

Other hypernyms of castle include hall, manse, mansion house, residence.

castle (noun)	mansion (noun)
interchanging the positions of the king and a rook	a large and imposing house
a large building formerly occupied by a ruler and fortified against attack	(astrology) one of 12 equal areas into which the zodiac is divided
(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

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