The evolution of caltrops

A caltrop is an antipersonnel weapon fashioned generally as a tetrahedron which will always have one point extending vertically. They have been in use since Medieval times, and are employed to stop or slow the advance of foot soldiers, war elephants, and especially camels.

The etymology of the word is from the Latin calcitrapa [foot+trap].

This device was used with great success by the Scots against the English at the Battle of Bannockburn in 1314 to disable horsemen. The Drummond clan deployed welded nail caltrops, halting English cavalry in its tracks and saving vital Scots infantry in a battle that resulted in 4,000 English casualities and Edward II retreating without shield and sword. Their use undoubtedly contributed to the resounding Scottish victory.

One of them was found in an archaeological dig at Jamestown. They have also been used in domestic disputes (labor strikes, environmental protests, etc) to disable vehicles. They were also deployed (by the criminals to slow pursuit) during last weeks helicopter-assisted robbery in Sweden.

The bottom one above has hollow spikes. Why?

"to puncture self-sealing rubber tires. The hole in the center allows air to escape..."

Clever. Photo and quote credit Wikipedia, where there is more info.