Catwoman uses caltrops as an anti-personnel weapon and bolas to entangle opponents at a distance. He dug entrenchments and placed caltrops in front of them, to slow down Muslim advances. He decided to lure the Persians out of the defenses of entrenchments and caltrops. Punji sticks perform a similar role to caltrops. Caltrops are still strung together and used as roadblocks in some modern conflicts. Tetrapods are concrete blocks shaped like caltrops, which interlock when piled up. The caltrops also hindered the speed of Muslim cavalry and infantry. It was possible to buy water caltrops in markets all over Europe until 1880. Contemporary caltrops look something like large jacks from the childhood game. During service in World War I, Australian Light Horse troops collected caltrops as keepsakes. Caltrops were used extensively and effectively during World War II. In modern times, special caltrops are also sometimes used against wheeled vehicles with pneumatic tires. Caltrops have been used at times during labour strikes and other disputes. Caltrops served to slow down the advance of horses, war elephants, and human troops. In modern times, caltrops are effective when used against wheeled vehicles with pneumatic tires.