Where the Castillo name comes from

Castillo is the Spanish word for castle, and as a surname it names one. Behind it is the Old Spanish castiello, a stronghold or fortress, and behind that the Latin castellum, which is a diminutive of castrum, a fort. So the name sits closer to the little fort than to the great hall. The Portuguese form of the surname is Castilho, and the Catalan castell is the same word again.