Because of the capital letter V in Várba, they don't just go to 'the castle', but to 'THE castle'. And in Hungary, 'THE castle' is Buda castle. Similarly, in Czech, if you would capitalize the letter H in hrad (castle), you would mean Prague castle or even more specifically, the office of the president of the Czech Republic, located in the castle complex. It is really tricky though, and an explanation should be provided here.
maybe they should at least accept buda castle as a translation if they give it as a hint