Specifically, I'm referring to the area where the War of the Ben'dr Mahk took place. I do not take the geography in Shadowkey as canon since it was a mobile game and its world map is innaccurate.

I ask because there isnt a lot of info on this area and I may be using it in a mod

Probably mountainos because of the Dragontail Mountains, althought Sentinel and Wayrest are on flatlands.

A mountain range, perhaps the dragontails, perhaps not, runs along the Hammerfell side of the Hammerfell-High Rock border, meeting up with the Druadach Mountains in
the Reach. The Reach geographic area extends into High Rock from Skyrim, and this is known as the Western Reach. The River Bjoulsae has its headwaters in this region as well. Plains are generally found north of the river as west from the West Reach.