The Spanish word 'castellano' (meaning 'Castilian', referring to the language and people of Castile) has its roots in Latin. It starts with the Latin word 'castrum' meaning 'fortified place', which developed into 'castellum' meaning 'castle' or 'fort' (essentially a diminutive form of castrum). From 'castellum' came the Latin adjective 'castellanus' meaning 'belonging to a castle'. The name of the region 'Castilla' (Castile) itself comes from the Latin 'castellum', likely due to the many castles in the region during medieval times. Finally, 'castellano' developed as the term for things relating to Castilla, including its language and people.

Several common Spanish words share this castle-related etymology. The word 'castillo' meaning 'castle' comes directly from Latin 'castellum'. 'Caserío' meaning 'hamlet' or 'group of houses' is also distantly related, sharing the root idea of a fortified settlement.

English speakers can recognize this etymology in several familiar words. 'Castle' comes from the same Latin 'castellum'. The word 'castellated', meaning 'built like a castle' or 'having battlements', also shares this origin. Even the name 'Chester' in English place names (like Manchester, Winchester) comes from the Latin 'castrum', referring to old Roman fortifications.