For some reason when looking at the walls tooltips, I didn't see the "+". Looking again, it's surely cumulative.
The tooltips could be written much more clearly. Since each tier of walls must be built on the previous tier, I see no reason not to just state something much more clear like "Increases outer strength to 200."
The tooltips could be written much more clearly. Since each tier of walls must be built on the previous tier, I see no reason not to just state something much more clear like "Increases outer strength to 200."