In one of the first two fight scenes, there is a (memorable) hit where one of the English gets his head bashed with a large hammer. Notice that when the English soldier is hit with the hammer, the blood splatters out all over the top of the helmet! Since when do helmets bleed?
When the hammer smashes the head the helmet wont protect you from the hammer. So the hammer hits the head and and cracks his head open. A helmet wont protect a blast like that.