The Spanish word 'castillo' (meaning 'castle') comes from the Latin word 'castellum', which meant 'castle' or 'fortress'. 'Castellum' itself was formed by combining the Latin word 'castrum' (meaning 'fort' or 'camp') with the diminutive suffix '-ellum'. So originally, a 'castellum' was literally a 'little fort'. This word evolved through Old Spanish 'castiello' before reaching its modern Spanish form 'castillo'.

A related Spanish word is 'castrense', meaning 'military' or 'relating to the army', which comes more directly from the Latin 'castrum'. You might also encounter place names in Spanish-speaking countries that include 'castro', referring to ancient fortified settlements.

Several English words share this Latin origin with 'castillo'. The most obvious is 'castle', which also comes from Latin 'castellum'. The English word 'castellated', meaning 'having battlements like a castle', is another related term. We also see this root in place names like 'Newcastle' and 'Lancaster' (which originally meant 'Roman fort on the River Lune').