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; cap. Burgos) and New Castile, to the south (now the region of Castilla-La Mancha, 30,589 sq mi or 79,225 sq km; cap. Toledo)
Sp. name Cas·tilla
A region and former kingdom of central and northern Spain. Autonomous from the tenth century, it joined with Aragon in 1479 after the marriage of Isabella and Ferdinand (1469), thus forming the nucleus of modern Spain.
Supposedly from the many castles constructed in the region.