castling vs castle

castling and castle both are nouns.

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

As nouns, castling and castle are synonyms defined as:

castling (noun)	castle (noun)
interchanging the positions of the king and a rook	interchanging the positions of the king and a rook
	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

castling (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