I know i can do both, I am trying to decide on what I think the better way to do it is, and what makes the most sense. I was thinking it would be nice to have a basic trench that lowers enemy offense, and then an upgrade, maybe a pillbox, that upgrades your defense.
I am more wondering if there is some reason that it tends to be what it is as opposed to just what it has always been. From a realism perspective, I would say all the way back to artillery in the original version, it would actually kind of make sense for it to lower defenders defense instead of attackers offense.
Of course I am making an xcom game based on an alien invasion, so it isn't like realism really matters. It isn't like a tactical half rpg, half shooter translates very well to a turn based strategy game anyway!