circummūnītĭo, ōnis, f. [circummunio];
in milit. lang., an investing of a town , circumvallation , Caes. B. C. 1, 19 fin. ; Auct. B. Hisp. 38 fin.
circummūnītiō ōnis, f circummunio, an investing, circumvallation : oppidi, Cs. ...
An Elementary Latin Dictionary