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 dispenses caltrops onto the ground every 4.4 seconds, similar to a Spike Factory. Each 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.


  • 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. Link for Caltrops Tips.
  • 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. 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 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, but they will deal 2 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.

Version History (BTD6)Edit


Nerf Caltrops attack speed decreased by 100% (every 2.2s --> 4.4s)

Buff Caltrops pierce increased by 100% (3 --> 6)



  • By default, Caltrops are dispensed at half the rate as a base Spike Factory.
  • One 0-0-0 Spike Factory costs twice as much as the Caltrops upgrade. The only game mode where this does not occur is Hard Difficulty, though this phenomenon is due to differences in rounding ($865 for a 0-0-0 Spike Factory as opposed to $860 for two $430 Caltrops upgrades).
  • 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.
  • 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 8 pierce each and providing 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.
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