Caltrop

[…] her father, the emperor Alexius, who reigned AD 1081-1118, ordered caltrops to be cast in front of his archers […]

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

Caltrops, tetrahedrons, and similar devices are designed to puncture vehicle tires or limit foot traffic. The standard design has four points.