As the Wikipedia article indicates, the term "Castilian Spanish" is used in lots of different ways. In the US, the way I've heard it used most often is to distinguish the Spanish spoken in most of Spain from that in Central and South America. However, there's also much variation among the varieties of Spanish spoken in Central and South America. People from one Central American country may have a different word for any number of different items than people from another Central American country. Even within a country, there is sometimes variation. The same thing is true of accent, the use of tú, and lots of other differences. I could have written an article very similar to the one in Wikipedia that focused only on differences in pronunciation, vocabulary, and grammar in Spanish speaking countries in the Americas.
The differences that the Wikipedia article mentioned are similar to those I referred to in my earlier response. The important thing is that people manage to understand each other even when they have different accents, use some different words, etc. When I was starting to learn Spanish, I was somewhat intimidated by the accent from Spain, that is, their pronouncing a soft c and a z as "th." But I now realize that it's no big deal. I listen to podcasts from Spain as well as from Latin America, and I find them equally challenging. I often find it harder to understand someone from Cuba or Puerto Rico than someone from Madrid.
All this is a longwinded way of saying that I do not think it depends on where you go in Spain. I think if you learn Spanish well, you'll be fine wherever you go in the Spanish-speaking world. The hard part is to learn Spanish well!