Caltrop

These weapons are also known as calthrops, jack rocks or star nails.  In medieval warfare, they were thrown into the path of enemy troops, either on foot or on horse back.  As they always landed with a spike pointing upwards they caused damage, slowing down an advance.   They have been used since Roman times, when they were known as "tribulae", up until the present day, when they are used to puncture tyres of vehicles.