castle vs mansion house

castle and mansion house both are nouns.

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

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

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

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