卡斯蒂利亚 transliterates 'Castilla / Castile'. The medieval Kingdom of Castile (
卡斯蒂利亚王国) merged with Aragon in 1469 through the marriage of Ferdinand and Isabella, forming the basis of modern Spain. The Castilian dialect became standard Spanish, which is why
卡斯蒂利亚语 (Castilian) is a synonym for
西班牙语 (Spanish).