I could be mistaken, but I'm thinking that HS doesn't do full damage in the area of the entire targeting circle, does it? That could explain your second observation.
I know that. The Hammer slam does x damage right where the hammer hits (or at least virtually hits) and y damage in the outer area (depending on the level). But that's not what I meant. If I assign the target to be dead center on the tower, the rook still might slam somewhere around the tower and not do full damage. But as I said, I am not sure if it does in fact not give full damage in such a case.
