Plot: Ambushing the enemy with weapons that stay undetected until their very strike can be an insidious, but efficient, military strategy. A team of weapons experts takes a look at three such inventions: the ancient Roman's caltrop, iron spikes spread across battlefields to bring horses and soldiers to their knees; the Dreyse needle gun, which changed the tactics of future armies; and the landmine, an age-old weapon lethally improved by modern technology