Caltrop

Chambers's Encyclopaedia, Volume 2: Beaugency to Cataract, p. 656

Caltrop, a four-pronged piece of iron, each prong about four inches in length, formerly thrown down in warfare to check the approach of the enemy's cavalry over a plain, or of besiegers in the ditch of a fortification.

A simple line drawing of a caltrop, which is a four-pointed star shape with a central circular base and four sharp, pointed prongs extending outwards.
Caltrop.
Source scan(s): p. 0669