[vlugorilla]

There is a bit of a controversy around this. And I don't say you are wrong. It's just that I personally consider that Castilian should not be used and does no longer exist. Here's why I think it like so:

Castilian originated as one of several Romance dialects in the Iberian Peninsula. It developed in the Kingdom of Castile during the Middle Ages, distinct from other regional languages like Catalan or Galician. With the unification of Spain, Castilian gained prominence, eventually evolving into modern Spanish. This was not merely a linguistic shift but also a result of political and cultural dynamics. The language we now call Spanish has absorbed influences from Arabic, indigenous languages of the Americas, and others, diverging significantly from its medieval Castilian origins. For this, Castilian has now disappeared, you just need to read how Castilian was written to see it has nothing to do with modern Spanish.

Today, Spanish is spoken by over 500 million people worldwide. In contrast, the Castilian region of Spain has a much smaller population (~3M). Referring to the language as Spanish acknowledges its extensive global presence and its modern version. Just as we refer to the language originating in Tuscany as Italian, not Tuscanian, calling the language from Castile 'Spanish' aligns with common linguistic naming conventions. Languages often take their names from the nations or cultural entities they are associated with, not their specific regions of origin.

Modern linguistic institutions, like the Real Academia Española, regard 'Castilian' and 'Spanish' as synonyms but recommend 'Spanish' for its inclusive and global character.

[Y_Y]

This is a nice and thoughtful post and I agree mostly. I'd like to add that my use of the word "Castilian" reflects my experience of usage of the term here in Barcelona (when speaking "Spanish", Catalan, and English). It's not a hard rule of course, but people are especially likely to refer to Castille over Hispania when distinguishing from other languages spoken historically within the country.

The term also usefully refers to the prestige dialect of Spanish, as might be spoken in Madrid. This is useful to distinguish from e.g. the "al-andalus" (Andalusian) spoken in the south which is more treated as a dialect than a separate language (though the distinction is of course fuzzy).

(On the other hand Barcelona in particular has a significant population of sudamericanos who will usually say "español", certainly that term is well used and understood.)