Castellum

 ( Roman Fortifications ) 

A castellumCharlton T. Lewis, Charles Short, A Latin Dictionary in Latin is usually:

• 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.
• 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".