As with the verses from the beginning of this chapter, it is unknown how a Nephite record obtained all these details. Certainly, some were learned after the fact, perhaps by spies or deserters. What becomes interesting is that, even if we cannot be sure of motivations, it is clear that the Lamanites were not immune to defections of their own. Just as some Nephites left the political designation of Nephite, so too did some of the Lamanites leave their cities and allegiances and create new ones. In this case, there is a new king.
This new king had his own army, and they placed themselves on a hill which was clearly defensible. The idea of making a stand on a hill was clearly known to Book of Mormon peoples, as both the Jaredites and Nephites also did that.