Caltrops

"Throws out spiked caltrops onto the track nearby."

- In-game description

 is the second upgrade of Path 3 for the Ninja Monkey in Bloons TD 6. It allows the Ninja Monkey to create an additional attack that dispenses caltrops onto the ground every 4.4 seconds (every 2.73s with Ninja Discipline), similar to a Spike Factory. Caltrop can pop up to 6 Bloons before it becomes used up. Caltrops have a lifespan of 70 seconds or 1 round, whichever comes first.

It costs $340 on Easy, $400 on Medium, $430 on Hard and $480 on Impoppable.

Summary
Purchasing the Caltrops upgrade adds secondary Caltrop throw acting very similar to Spike Factory. Because of the potential to stockpile spikes, it is a very useful low-cost option for countering Camo Bloons in earlier rounds, as well as adding up extra overall damage in the early mid-game. Contrary to popular belief, Caltrops is highly beneficial with Alchemist buffs, as spike piles will always retain buffs provided that they are dispensed while the original tower gained the buffs.

This upgrade basically allows Ninja to hybridize Spike Factory with its own primary attacks. The caltrop attack can benefit from Ninja Discipline to dispense caltrops at a faster rate, but with any other upgrade it generally works independently as a crosspath otherwise.

Tips

 * Caltrops that are dispensed while under influence of Stronger Stimulant will give such Caltrops +3 pierce and +1 damage, giving each Caltrops a total of 9 pierce and 2 damage.
 * Note that every Caltrops pile does consume one unit of Acidic Mixture Dip or Berserker Brew/Stronger Stimulant.
 * Unlike popular belief, Caltrops does not actually slow down the Ninja Monkey's main shuriken attacks, as the caltrops throwing animation does not at all interfere with the rate of shuriken throwing. This makes the Caltrops upgrade not a bad choice for some extra popping power.
 * 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 Level 11+ 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, especially so due to the massively increased attack speed and pierce of every attack.
 * Apparently, Grandmaster Ninja crosspathed with Caltrops will still only have 5 pierce after Version 12.0 and still only deal 1 damage. This is likely a bug, as Caltrops is meant to have 6 pierce each. Regardless, Caltrops is still a good upgrade to pair with Grandmaster Ninja.
 * Unlike Spike Factories, Caltrops cannot ignore Line of Sight, which means the Ninja Monkey needs to directly see bloon track in order to dispense any caltrops around any visible bloon track.

Trivia

 * The Caltrops upgrade redesign in Version 12.0 indirectly nerfed the Grandmaster Ninja + Permanent Brew 2TC combo, as each caltrop from the 5-0-2 Grandmaster Ninja overall does less DPS than normal. In particular, there are fewer numbers of caltrops spawned at once, thus benefiting less from the +2 pierce per projectile from the Grandmaster Ninja and +3 pierce per projectile from Permanent Brew. This has made the combo impossible to do as of said version.
 * When Grandmaster Ninja was crosspathed 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 between Version 9.0 and 11.0, but 8 pierce each in Version 12.0. This provided more value with damage buffs.
 * This has since changed after the rework of Grandmaster Ninja, thereby leaving the Caltrops pierce at 5 instead of 6, nerfing the Caltrops attack.
 * The upgrade icon resembles the Long Life Spikes upgrade for the Spike Factory.