Caltrops are ancient military devices with four sharp spikes arranged to ensure one always points upward.

Historically, they have been used in warfare for centuries, with evidence of their use dating back to antiquity. 

The primary purpose of caltrops is to create obstacles on the battlefield, they were scattered on the ground during battles or sieges, obstructing the advance of enemy forces. Caltrops were particularly effective against cavalry charges, as they could cause horses to stumble and throw their riders off balance, disrupting the momentum of the attack.

Despite their simplicity, caltrops remained a potent weapon on the battlefield for centuries. Even in modern times, variations of caltrops have been used by military and law enforcement agencies for crowd control and perimeter defense. While their role in warfare has diminished with advances in technology, the basic concept of the caltrop continues to influence the design of defensive barriers and anti-personnel devices today.