> Castilian (castellano), that is, Spanish, is the native language of the Castilians. Its origin is traditionally ascribed to an area south of the Cordillera Cantábrica, including the upper Ebro valley, in northern Spain, around the 8th and 9th centuries; however the first written standard was developed in the 13th century in the southern city of Toledo. It is descended from the Vulgar Latin of the Roman Empire, with Arabic influences, and perhaps Basque as well. During the Reconquista in the Middle Ages, it was brought to the south of Spain where it replaced the languages that were spoken in the former Moorish controlled zones, such as the local form of related Latin dialects now referred to as Mozarabic, and the Arabic that had been introduced by the Muslims. In this process Castilian absorbed many traits from these languages, some of which continue to be used today. Outside of Spain and a few Latin American countries, Castilian is now usually referred to as Spanish.
> In Spain and in some other parts of the Spanish-speaking world, Spanish is called not only español but also castellano (Castilian), the language from the Kingdom of Castile, contrasting it with other languages spoken in Spain such as Galician, Basque, Asturian, Catalan, Aragonese and Occitan.
> The Spanish Constitution of 1978 uses the term castellano to define the official language of the whole of Spain, in contrast to las demás lenguas españolas (lit. "the other Spanish languages").
It's the same language. I'm a Spaniard, so I know it well. Name it the way you'd like, it can be called Spanish, Español or Castellano everywhere from Mexico to Patagonia, and from The Canaries up to the Pyrenees.
"name it the way you'd like, it can be called Spanish" is a very different proposition to "[you should say] Spanish, Castilian does not exist [and you are wrong to use that name]", which was the angle of the poster who kicked all this off.
> from Mexico to Patagonia, and from The Canaries up to the Pyrenees.
Sounds a bit imperialistic?
Notwithstanding the tens of millions of native speakers of autochtone non-spanish languages in these territories: Mapuche (260K), Quechua (7.2M), Aymara (1.7M), Guaraní (6.1M), Wayuu (400K), Mayan (6M), Miskito (150K), Garifuna (120K), Nahuatl (1.7M), Mixtec (530K), Catalan (4.1M), Basque (750K), Galician (2.4M). Spanish is quickly eroding all of these, but they still exist! (And this only counts native speakers. The number of people who are fluent in Guarani or Catalan is certainly more than the double of that.)
> Not imperialistic. Would you say the same of the English language too?
Yes, of course? When I think of imperialism the first thing that comes to my mind is precisely "the bri'ish empi'ah and its commonwealth"! If English is currently the world's default language is just because of the triumph of English/American imperialism.
> Castilian (castellano), that is, Spanish, is the native language of the Castilians. Its origin is traditionally ascribed to an area south of the Cordillera Cantábrica, including the upper Ebro valley, in northern Spain, around the 8th and 9th centuries; however the first written standard was developed in the 13th century in the southern city of Toledo. It is descended from the Vulgar Latin of the Roman Empire, with Arabic influences, and perhaps Basque as well. During the Reconquista in the Middle Ages, it was brought to the south of Spain where it replaced the languages that were spoken in the former Moorish controlled zones, such as the local form of related Latin dialects now referred to as Mozarabic, and the Arabic that had been introduced by the Muslims. In this process Castilian absorbed many traits from these languages, some of which continue to be used today. Outside of Spain and a few Latin American countries, Castilian is now usually referred to as Spanish.
[1] https://en.m.wikipedia.org/wiki/Spanish_language