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Creates temporary sentry guns and deploys them nearby.
~ In-game description

Sentry Gun is the first upgrade of path 1 for the Engineer Monkey in Bloons TD 6. It allows it to create sentry guns within its range every 10 seconds. Each sentry lasts for 25 seconds (lasts 30 seconds with Vigilant Sentries MK), meaning on average up to between 2 and 3 sentries from the same Engineer become active at a time. The creation of sentries is affected by attack speed buffs such as Jungle Drums Villages. If crosspathed with Cleansing Foam, newly produced sentries can also get Cleansing Foam effect each hit to strip Camo and Regrow as well as cleanse Leads in a similar way to normal foam.

It costs $425 on Easy, $500 on Medium, $540 on Hard, and $600 on Impoppable.



Engineers with the Sentry Gun upgrade receive green-colored nailguns and a wrench. The wrench is a combination wrench colored dark grey with one light grey marking at the jaws and two light grey markings at the end. Most Engineers with the Sentry Gun crosspath hold the wrench with their left hands, while Double Gun and Bloon Trap hold them in their backs. Engineers creating sentries use a unique deployment animation.

Type of Sentry[]

Nail Sentry[]

Nail Sentry

Nail Sentry, referred in-game as "Sentry", is an green-colored automatic sentry that shoots nails. It shoot nails every 0.95 seconds, and each nail can pop 2 bloons (3 with Oversize Nails) and they have a range of 45 (+4 with Larger Service Area). The nails are of the sharp damage type and cannot pop lead or frozen without special upgrades. Crosspathing with Oversize Nails changes the sentry damage type from sharp to shatter (sharp + frozen-popping).

  • Damage Type: Sharp Type (pops Black, White, Purple) Sharp
  • Damage: 1
  • Pierce: 2 (3 with 1-0-1)
  • Attack Speed: 1.05x (every 0.95s)



Sentry Gun is traditionally the main feature of the Engineer that gives him the edge at early-game, and it remains true in BTD6. In essence, the Sentry Gun upgrade provides the Engineer with an attack that allows him to deploy nearly statistically equivalent 0-0-1 Dart Monkeys nearby. These sentries give the Engineer with considerable early-game damage, especially while combined with the fair attack speed and pierce of the base Engineer himself.

Unlike their BTD5 counterpart sentries do not have footprint that blocks placement of towers. Therefore, towers can be placed freely nearby sentry spots without getting confronted with occupied placement of sentries. On that other note, sentry spots conversely become blocked from being placed on top of existing towers, unless in the extreme case where almost all available land space becomes unavailable where these sentries will only spawn onto bare crevices between towers. Such interactions can be used to deliberately alter where the Engineer can place sentries.


  • Sentries spawned from the Engineer will automatically be set to whatever the Engineer's target priority is. They can also be manually changed to a specific target priority.
  • While the Sentry Gun upgrade helps bolster popping power for the Engineer, it is worth upgrading further to Faster Engineering for further DPS, as increasing the production speed further is far more effective than just the range or pierce crosspathing in most situations.
  • Note that adding Sentry Gun effectively increases the attack range coverage of the Engineer Monkey somewhat, although this is dependent on the where the Engineer's sentries may land.

Version History[]

When first released, sentries had very little popping power, especially in relation to 0-0-0 Dart Monkeys, which now have 2 pierce instead of 1. The base sentry pierce was increased back to the BTD5 amount to compete with the value of 0-0-0 Dart Monkeys. Additionally, Sentries now have crosspathing benefits in later updates.

12.0 (compared to BTD5)
  • Nerf Pierce of Sentry Gun decreased (2 → 1).
  • Buff Standard Sentry pierce increased from 1 to 2.
  • Buff 110 Larger Service Area now applies 1/5th the range (+4) to any sentries produced.
  • Buff 101 Oversize Nails allows sentries produced by this tower to pop frozen Bloons.
  • Buff 101 Oversize Nails now increases sentry pierce by +1.

Ninja Kiwi stated in the 20.0 Patch Notes that the reasoning behind these changes was "continuing our efforts to make crosspaths meaningful" and to "fit the style of each path more solidly".

  • Buff 120 Deconstruction now applies to sentries.
  • Buff 130 Cleansing Foam now applies to sentries.

Bug fixes[]

Mostly just fine-tuning for special cases.

  • [Bug Fix] 110 Larger Service Area now increases Sentry deploy range.
  • [Bug Fix] Nerf Fixed a bug where Bloon Trap granted camo detection and lead poppage to sentries.
  • [Bug Fix] 1xx Sentry Gun sentries no longer gain camo detection when a save is loaded.
  • [Bug Fix] 1xx Sentry Gun sentry placement will no longer ignore 'smart placement' if the Engineer cannot reach the track.


When a sentry is placed:


Official artwork[]


  • Before the sentry pierce buff in Version 15.0, Sentries were the slowest one-projectile towers to have only one pierce, excluding Sniper Monkeys (their bullets are too fast to be visibly moving in-game).