Sentence:  ... her father, the emperor Alexius, who reigned AD 1081-1118, ordered <WSD>caltrops</WSD> to be cast in front of his archers ... 
Meaning of the word marked with <WSD> tags:  (weaponry) A small, metal object with spikes arranged so that, when thrown onto the ground, one always faces up as a threat to pedestrians, horses, and vehicles.