Castellum

1. Limes watchtower, Germany (reconstruction)

2. Ruins of a castellum divisorium, France

A castellum^[1] in Latin is usually:

a small Roman fortlet or tower,^[2] a diminutive of castrum ('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 Germanic provinces.
a distribution, header and settling tank in a Roman aqueduct or castellum aquae.

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

References

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