Castellum

a small isolated fortress, or one of a series of such fortresses, of the ancient Romans.
Historical Examples

This castellum is a miniature embodiment of Roman taste and skill in architecture.
The Actress in High Life Sue Petigru Bowen

It was then a question which word should prevail,—tour (turris), or chastel (castellum).
The Story of London Henry B. Wheatley

Then we shall be able to invest the castellum closely, leaving a numerous body to prevent any sortie.
Annals of a Fortress E. Viollet-le-Duc

I doubt whether the walls of any Roman castellum could have resisted such a ram.
My Autobiography F. Max Mller

The site was originally occupied by a Roman ‘castellum’ which the Visigoths also used as a citadel.
Legends, Tales and Poems Gustavo Adolfo Becquer

In the north-east s before t is still retained—Walloon chestai (chteau, castellum), fiess (fte).
Encyclopaedia Britannica, 11th Edition, Volume 11, Slice 1 “Franciscans” to “French Language” Various

Under Drusus a castellum was erected here, which was destroyed by the Franks and the Alemanni.
The Cathedrals and Churches of the Rhine Francis Miltoun

“There were giants in those days,” said Lady Mabel, gravely, gazing on the castellum.
The Actress in High Life Sue Petigru Bowen

On a jutting crag of rock, in plain view from the town below, is an ancient Roman castellum.
Italian Highways and Byways from a Motor Car Francis Miltoun

Every castle that they took was burned and destroyed, like Sir Ingelric’s castellum at Speen.
A Legend of Reading Abbey Charles MacFarlane