Alma begins his preaching to one set of people, and will change to address a new set of people who have just arrived. The text does not give us enough information to distinguish between the two, except to note that these newcomers were the poorest among them. That seems to indicate that the first group of people were not of the poorest class. Nevertheless, the physical setting of the hill suggests that this is countryside, and not city. While the first group does not appear to have been the poorest, nor would we expect that they would be of the highest class, for those people would more likely have remained in their city.
Cultural: Alma speaks to people on a hill because it allows him to be seen and heard more readily by a larger congregation. From the standpoint of a public presentation, it is the same principle as stadium seating, but in reverse. The placement of a speaker at a lower position, with the audience rising above is a more logical arrangement, as the audience can more easily see over those in from of them. While the verse does not specifically say that Alma was above his audience, it is highly probable that Alma was closer to the top of the mountain than those who were listening.
The reason for this situation is as much cultural as practical. The pragmatic reasons might be, in addition to visibility, the greater ease of a single person climbing that a larger congregation. In addition to any other benefit, there are some cultural associations that would suggest this as the favored arrangement. The first is that it would more accurately reflect the use of the temple pyramids as presentation platforms. In the temple ceremonies in their cities, the presenter/speaker would walk up the pyramid, and the congregation would be gathered together in the temple’s courtyard. Thus Alma might ascend to hill as a parallel to ascending the pyramid were he in the city.
There is another association that is deeper than the imitation of the city’s arrangement for speaking. The conceptual nature of mountains in Mesoamerica was as a sacred location that allowed communication between the layers of the world; heaven, surface, and underworld. This conceptual understanding of a mountain as an inherently sacred space was also present in the Old World, and underlies the very creation of pyramids as artificial mountains. For Alma, the cultural reasons for ascending a hill would have included this conception of sacred things being spoken in a naturally sacred location.
Of course, we also have the conceptual difference between being in the artificial city and natural outside in the natural land. The typical class of people found in each of these locations would be similarly different as the different classes depended on each type of setting for their livelihood. The elite were in the city, the farmer-poor were in the countryside. Alma is preaching to the farmer-poor in the country, in their own territory, and away from the setting in which their subservience to the elite is manifest in nearly every stone of every city building.
Geographic: Alma is preaching on a hill named Onidah. This is a name that we will see again in the Book of Mormon in Alma 47:5. This latter is not listed as a hill, but as a place of refuge “Alma 47:5 … therefore they fled to Onidah, to the place of arms.” Reynolds and Sjhodahl note the similarity of names and the possibility that they might be the same, but suggest that they are different locations ((George Reynolds and Janne M. Sjodahl, Commentary on the Book of Mormon, edited and arranged by Philip C. Reynolds, 7 vols. [Salt Lake City: Deseret Book Co., 1955-1961], 5: 48.)
In the context of Alma 47, the “place of arms” might appear to be an armory of some sort, as the Lamanites flee to that location after the threat of a military attack. Clearly that it is not a possibility for the hill on which Alma is preaching. He appears to be on a physical hill, and the only connection to “a place of arms” is the name itself.
The settings for the two Onidahs do suggest that they are separate locations. The one discussed in Alma 47 is not very far from Lehi-Nephi in the heart of Lamanite country, and the Hill Onidah is in Nephite territory, even though it is bordering wilderness where there are Lamanites. Sorenson’s suggestion is that the hill Onidah might be so named because it is an outcropping of obsidian, a material much used in Mesoamerican cutting instruments, including weapons of war ((John L. Sorenson, An Ancient American Setting for the Book of Mormon [Salt Lake City and Provo: Deseret Book Co., Foundation for Ancient Research and Mormon Studies, 1985], 252.)