region & former kingdom in N and central Spain: gained autonomy in 10th cent. & united with León, & later with Aragon (15th cent.), & became the nucleus of the Spanish monarchy: traditionally divided between Old Castile, to the north (now the region of Castilla-León, 36,350 sq mi or 94,147 sq km, pop. 2,546,000; cap. Burgos) and New Castile, to the south (now the region of Castilla-La Mancha, 30,589 sq mi or 79,225 sq km, pop. 1,658,000; cap. Toledo)