When Amalickiah went forth to "compel" the Lamanites to arms, the remainder of the Lamanite army fled to a "place which was called Onidah" (Alma 47:5). Here the abridger Mormon gives the reader a clue as to why the Lamanites fled there---it was "the place of arms". It is hard to tell what this phrase means. It might mean a location where weapons were either made or stored.
According to John Sorenson, in Mesoamerica, what constitutes a "place of arms" is obvious; it can hardly be anything other than an obsidian outcrop. This volcanic glass was the most convenient, most effective, and cheapest substance for manufacturing arms or any cutting tools. (Note that Alma 49:2 informs us that "arrows and stones" were the chief weapons of the Lamanites.) Trade in obsidian was the mainstay of commerce from earliest times. Some routes over which it moved extended as much as 700 miles. It happens that one of the most extensive sources of this key material is the hilly zone called El Chayal, approximately sixteen miles northeast of Kaminaljuyu. Spots within the kilometers-wide obsidian exposures at El Chayal are virtually paved with waste chips, where cutting implements have been shaped by chipping. Obsidian from El Chayal was exported widely as early as Jaredite times. El Chayal is not the only possible obsidian source near Kaminaljuyu, as these references make clear, but it is the most likely one. [John L. Sorenson, An Ancient American Setting for the Book of Mormon, F.A.R.M.S., pp. 252, 385]
Geographical Theory Map: Alma 47:1 Amalickiah Goes to the Land of Nephi (19th Year)
Lamanite Army Goes to the Place Onidah (19th Year)