A castellum in Latin is usually:

• a small Roman fortlet or tower, a diminutive of ('military camp'), often used as a watchtower or signal station like on Hadrian's Wall. It is distinct from a , which is a later Latin term that was used particularly in the Germanic provinces.

• a distribution, header, and settling tank for municipal water in a Roman aqueduct (known as a castellum aquae/castellum divisorium).

It is the source of the English word "castle".

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