The plural form of castello (Italian/Spanish for castle); also a variant Latin plural referring to multiple castles or fortified strongholds.
From Italian 'castello' or Latin 'castelli' (plural of castellum). The term appears in historical documents describing multiple fortifications or in place names throughout medieval Europe, particularly in Italy and Iberia.
When you see Italian city names like 'Castellammare' (castle on the sea) or Spanish regions with 'Castilla' (land of castles), you're looking at linguistic relics of when medieval lords controlled multiple 'castelli' across a region!
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