A castellum[Charlton T. Lewis, Charles Short, A Latin Dictionary] in Latin is usually:
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a small Ancient Rome fortlet or tower,
[C. Julius Caesar, Gallic War; 2,30] a diminutive of castra]] ('military camp'), often used as a watchtower or signal station like on Hadrian's Wall. It is distinct from a burgus, which is a later Latin term that was used particularly in the Germania provinces.
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a distribution, header, and settling tank for Water supply in a Roman aqueduct (known as a castellum aquae/ castellum divisorium).
It is the source of the English language word "castle".