Well, I used to play a lot of Battle for Wesnoth, and I thought it might be interesting to see the following concept in BattleMaster: particular unit types automatically fight better or worse in particular terrain types.
Rural: The baseline; all units are equally advantageous.
Townsland: All units are equally advantageous.
City: Infantry have a slight advantage over other unit types here.
Woodlands: Cavalry has a slight penalty here; archers fare better.
Mountains: Nearly every type of unit fares poorly here, particularly cavalry.
Badlands: Cavalry have a slight advantage here.
Stronghold: All units fight the same here due to the effects of the stronghold.
We can squabble over the particular details; the examples are just to give you an idea of what I'm saying... although it just occurred to me that we really don't have a big enough variety of terrains for this suggestion to be useful.
I would do things a little differently:
Rural: Cavalry movement bonus.
Badlands: Cavalry movement bonus.
Woodlands: Cavalry has a slight movement penalty here; archers fare better. Infantry cohesion penalty. Archer defensive bonus.
Mountains: Nearly every type of unit fares poorly here. Cavalry move at same rate as infantry. Archer defensive bonus.
Townsland: Cavalry movement penalty. Infantry cohesion bonus. Archer defensive bonus.
City: Cavalry movement penalty. Infantry cohesion bonus. Archer defensive bonus.
Stronghold: Cavalry movement penalty. Infantry cohesion bonus. Archer defensive bonus.
