Caltrops is the second upgrade of Path 3 for the Ninja Monkey in BTD6. It allows the Ninja Monkey to create an additional attack that dispense caltrops every 2.2 seconds. Each caltrop can pop up to 3 Bloons. It costs $340 on Easy, $400 on Medium, $430 on Hard, and $480 on Impoppable.
Unlike popular belief, Caltrops does not actually slow down the Ninja Monkey's main shuriken attacks, making it not a bad choice for some extra popping power. Link for Caltrops Tips.
Confirmed by this Reddit post about Caltrop Tips, Caltrops that are on the track will not wear out its Acidic Mixture Dip if the original tower's Acidic Mixture Dip wears out. In fact, Caltrops that were affected by Acidic Mixture Dip will retain their bonuses until consumed or destroyed.
Caltrops Ninjas nearby Obyn will become much more powerful, which allows them to be deploying extra sharp caltrops with +2 pierce. Additionally, Shinobi Tactics Ninjas in huge groups will cause nearby Caltrops Ninjas to produce massive caltrop piles, which in turn deal massive overall stockpiled on-track damage.
When Grandmaster Ninja is crossed path with Caltrops after the 9.0 balance changes, the +2 pierce bonus for the shurikens also applies to the caltrops, making them have 5 pierce each and providing more value with damage buffs.