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".