|This article is part of the series on:
Military of ancient Rome (portal)
753 BC – AD 476
|Roman army (unit types and ranks, legions, auxiliaries, generals)|
|Roman navy (fleets, admirals)|
|Lists of wars and battles|
|Decorations and punishments|
|Military engineering (castra, siege engines, arches, roads)|
|Strategy and tactics|
|Frontiers and fortifications (limes, Hadrian's Wall)|
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.