The Calthrop monster stopped the invading army. Since ancient Greece the antipersonelle device called a calthrop (caltrop, caltrap, cheval) has been used to slow down advancing armies. Four sharp nail like points in a tetrahedon, a calthrop always has one foot-piercing point sticking straight up. These nasty little militarized jacks could stop infantry, calvary, chariots, war elephants, and camels.