Sentence:  By Sung times, several different types of <WSD>caltrops</WSD> had been developed. As in earlier times, both caltrops could be made from both wood and iron... 
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.