Here the original manuscript reads those. When Oliver Cowdery copied this passage into the printer’s manuscript, he initially wrote these but then corrected these to those. Nonetheless, the 1830 compositor ended up setting these, which has continued in all subsequent editions. For a list of instances where the 1830 compositor mixed up those and these, see under Mosiah 28:1. For instances where Oliver Cowdery mixed up these two demonstratives, see under Alma 3:25.
Here at the beginning of Alma 50, there is a long narrative chain that refers to various defensive works for which those is consistently used as a link between the works:
The use of those throughout this passage is clearly intended. Thus the one inconsistent change of those to these at the beginning of verse 2 should be reversed.
Summary: Restore in Alma 50:2 the original those (“upon the tops of those ridges of earth”).