A Caltrop, or Tribulus, is a weapon made up of two or more sharp nails or spines arranged in such a manner that one of them always points upward.
They would often be hidden in grass or fords as part of defences that slowed the advancement of marching troops and horses, especially those drawing chariots. It was common for them to be used in areas around forts.
The name “caltrop” comes from the Latin calcitrapa (foot-trap).
The synonymous Latin word “tribulus” provides part of the modern scientific name of a plant commonly called the caltrop, whose spiked seed case can injure feet and even puncture tyres.