Calthrop

Small device scattered on the ground to injure and make any passing horses lame (or later to puncture rubber tyres). It has four metal spikes forming the four corners of a tetrahedron so that one metal spike will always points upwards. Manufactured by setting the spikes in an iron ball, or by simply joining the spikes together.

1) A small spiked device, intended to impede horses.

(Prestwich, Michael. Armies and Warfare in the Middle Ages: The English Experience, 347)

2) Small metal ball with four (angled) projecting spikes placed on battlefield to maim horses.

(Seward, Desmond. Henry V: The Scourge of God, 222)

small four pointed metal weapons that were strewn on the ground. They were designed in a way that one point would always be straight up. This would be a danger to enemies and in particular horses.