Castilian or Castellano is just the more precise name for the language that most people call Spanish, and as such it’s spoken throughout Spain and Spanish-speaking countries in Latin America and elsewhere. In Spain there are three other main languages and a few tiny pockets of other minority languages. Gallego is like Portuguese and is spoken in the far North West, Basque is spoken in the Basque Country (obviously) and Navarra, although it’s so difficult that lots of Basques don’t speak it very well. The one to really look out for is Catalan, which is spoken in Catalunya, the Balearic Islands and parts of Valencia, France and Sardinia, with between 5 and 10 million speakers, depending on who is quoting the statistics. This really is the first and dominant language in many areas and it’s said you can go right through school and University in Catalunya without studying any Spanish. I think you’re right to avoid Catalan-speaking areas unless you plan to stay for the long term - not because there’s anything wrong with them (on the contrary, they’re fabulous), but if you want to learn Spanish properly, it will make things difficult and confusing. So for pure Spanish, you’re left with everywhere else - i.e. most of inland Spain and the Med coast from the Portuguese border up as far as Valencia (with the Catalan/Spanish border being quite vague and porous around there), most of the North including the coast in Cantabria and Asturias, and the Canaries.
Within Castilian Spanish areas, you naturally have accents, as in English. In Andalucia, Extremadura, Murcia and the Canaries, pronunciation and slang is distinct from standard Spanish (more like Latin American), but is not so different as to make things too difficult. The equivalent of “Home Counties” or “BBC” English is probably found most around Salamanca, Burgos, Leon - cities with universities, cathedrals and often old-fashioned ways and manners.
