Spanish vs Castilian

Spanish and Castilian both are nouns.

Word	Noun	Adjective	Verb	Adverb
Spanish	Yes	Yes	No	No
Castilian	Yes	No	No	No

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

Spanish: the Romance language spoken in most of Spain and the countries colonized by Spain
Castilian: the Spanish language as spoken in Castile

Other hyponyms of Spanish include Castilian language, Judeo-Spanish, Ladino, Mexican Spanish.

Spanish (noun)	Castilian (noun)
the Romance language spoken in most of Spain and the countries colonized by Spain	the Spanish language as spoken in Castile
the people of Spain	a native or inhabitant of Castile

Spanish (adjective)	Castilian (adjective)
of or relating to or characteristic of Spain or the people of Spain