In Alma 49:18 we find that "the Lamanites could not get into [the Nephite] forts of security by any other way save by the entrance, because of the highness of the bank which had been thrown up, and the depth of the ditch which had been dug round about, save it were by the entrance." According to Michael and June Hobby, we can compare the descriptions in the Book of Mormon with descriptions of the Maya site of Becan, located in Campeche, Mexico (Mesoamerica). The report quoted is "Defensive Earthworks at Becan, Campeche, Mexico, Implications for Maya Warfare" by David L. Webster. Webster reports:
Figures derived from our 40 mapping profiles indicate that the vertical distance from the present top of the parapet to the original ditch bottom . . . averages 9.6m. A conservative estimate of the reduction in height of the parapet is put at 2m. Thus when the defenses were first erected, an attacking enemy would have been confronted by a barrier (measured from the bottom of the ditch to top of parapet) about 11.6m high (about 35 ft).
[Michael and June Hobby, Proofs of the Book of Mormon, p. 38]