Wiktionary
castellum
n. (context historical English) A small Roman detached fort or fortlet used as a watch tower or signal station.
Wikipedia
Castellum
A castellum in ancient Latin is usually either:
- a small Roman fortlet or tower, a diminutive of castrum (" military camp"), often used as a watchtower or signal station e.g. on Hadrian's Wall. It should be distinguished from a burgus, which was a later Latin term used particularly in the Germanic provinces.
- a settling or storage tank in a Roman aqueduct.
It is the source of the English word " castle".
Usage examples of "castellum".
It was not a camp, sir, a castrum, but a castellum, a little camp, or watch-station, to which was attached, on the peak of the adjacent hill, a beacon for transmitting alarms.
The old tower Psephina, in the middle ages Neblosa, was named Castellum Pisanum, from the patriarch Daimbert.