Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
circum -mūnĭo (old orthog. cir-cummoenĭo, Plaut. Capt. 2, 2, 4), īvi, ītum, 4, v. a., to wall up around, to fortify, secure (freq. in the histt., elsewhere rare; but in MSS. constantly confounded with circumvenire; cf. Oud. ad Caes. B. C. 1, 18; 1, 81; 1, 84; 2, 16; 3, 97; Schneid. ad Col. 5, 9, 11; 5, 10, 1): plantas caveis, Col. 5, 9, 11: oppidum, Auct. B. Afr. 79: Thapsum operibus, id. ib. 80; cf. Auct. B. Hisp. 38: aliquos ut feras, Caes. B. C. 1, 84: (hostes) vallo fossāque, id. ib. 1, 80: crebris castellis circummuniti, id. B. G. 2, 30; Plaut. Capt. 2, 2, 4.
Absol., Auct. B. Hisp. 34 fin.