For example, Barrage: +15% strength when attacking enemies in rough terrain like forest and hills.
The question is, does it mean: 1) I'm initiating an attack when standing on a rough terrain or 2) The enemy is standing on a rough terrain when I'm attacking it?
Google-fu doesn't bring up any result other than a comprehensive list of possible promotions, which for some reason lack explanations like the one I'm looking for
Best Answer
Number 2. Terrain promotions are based on where the defender is standing so barrage will give you a bonus attacking into hills/forest.
For melee promotions, this also applies to defending. If a unit with Drill is standing in a forest/hill, it will get a bonus to its combat strength if it is attacked.