A counter attack is launched against the Gadianton which fails. The Nephites would have sent a reasonable force, and that reasonable force was beaten back. This very fact tells us that the location of the Gadiantons in the mountains was fortified. The Gadiantons were no guerilla robbers who faded into the mountains, not to be found.
(Daniel C. Peterson. “The Gadianton Robbers as Guerilla Warriors.” Warfare in the Book of Mormon. FARMS, Salt Lake City, 1990, pp. 146-173 presents the argument that the Gadiantons were indeed, such guerilla warriors. Peterson is reading the text for the more direct surface meaning rather than looking below the level of Mormon’s descriptions. Thus he does not deal with the implications of the descriptions as much as he used the descriptions themselves. For instance, he takes the terms “plunder and rob and murder” at their face value rather than as code words for the Gadianton establishment of tribute-relations through military action. Obviously the interpretation used in this commentary requires extra cultural content, and a recontextualizing of those terms. However, as has been argued, the descriptions of the events fit best into the Mesoamerican tribute pattern, particularly when the Gadiantons exercised the same termed actions when they controlled Zarahemla.)
They actively beat back the Nephites. This tells us that there was an engagement, and the “army of strong men” sent by the Nephites was insufficient. The Gadiantons must therefore also have their own army of strong men, and most likely have a fortified location. They must have built a city.