Castellum

Reconstructed Limes watch tower, between Bad Nauheim and Pfaffenwiesbach, Germany.

Ruins of the castellum divisorium in Nîmes, France, which distributed water from an aqueduct to individual water pipes.

A castellum is a small Roman detached fort or fortlet used as a watch tower or signal station. The Latin word castellum is a diminutive of castra ("military camp"), which in turn is the plural of castrum ("watchpost"); it is the source of the English word "castle".

The term castellum was also used to refer to a settling or storage reservoir along the length of an aqueduct.