Trolls can't walk over a moat, dry or wet. (Hint: they can't walk across a single core wood beam used as a bridge either) If you dig one far enough away from the tower, the log wielding trolls can't reach it. During the Viking age logs were used by groups of men as battering rams to breach stone walls and iron reinforced gates. It makes sense that a creature that is strong enough to wield a log as a weapon could use it as a battering ram to breach stone walls.