Creates temporary sentry guns and deploys them nearby.
~ Official Bloons TD Battles Mobile description.

Sentry Gun icon as seen in BTD6

Sentry Gun is the first upgrade of path 1 for the Monkey Engineer in Bloons TD 5 Deluxe, Bloons TD 5 Mobile, Bloons Monkey City, Bloons TD Battles Mobile, and Bloons TD 6 that allows it to create sentry guns within its range. The Sentry Guns shoot nails, and each nail can pop 2 bloons. It is very useful for the early rounds as it acts like several Dart Monkeys all attacking at once. The Monkey Engineer can create up to 3 Sentry Guns with no fire rate upgrades when upgraded to this. However, the Engineer can produce more sentries if it's under the influence of other towers such as Jungle Drums Villages or additional upgrades such as Fast Engineering/Faster Engineering.



  • Sentry Guns can be sold for $0, and also have a target priority which can be changed. Often Sentry Guns would only be sold to give space for another tower to be placed on.
  • They are effective on daily challenges and special missions where the amount of towers is limited, as they do not affect the total tower count.
  • Sentry Guns are created every 10 seconds and have a lifespan of 25 seconds.
  • In the case of BTD5 Deluxe, Bloons Monkey City Flash and BTD6, Sentries' target priority is determined by the Engineer who has spawned them. In BTD5/Battles/Monkey City Mobile, however, Sentries default to First no matter what target priority the Engineer has.


  • Similar to BTD5, Sentry Guns can be sold for $0. Fortunately, Sentries don't have Footprints and can allow towers to be stacked on top of them.
  • They are helpful in Daily Challenges and Advanced Challenges where the amount of towers is limited, as they do not affect the total tower count.
  • Sentry Guns are created every 10 seconds and have a lifespan of 25 seconds.
    • Lifespan increases to 30 seconds with necessary MK.
  • 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.

Version History (BTD6)Edit

12.0 (compared to BTD5)

Nerf Pierce of Sentry Gun decreased (2 --> 1)

  • [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
  • Buff Standard Sentry pierce increased from 1 to 2
  • [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
  •  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".



  • When you upgrade the Engineer to Sprockets, any new sentries spawned will have the name "SentrySprockets". These sentries are not affected by Monkey Knowledge in BMC Flash.
    • In the mobile versions, the sentries are instead internally upgraded 0-3, and are affected by Knowledge in BMC Mobile.
  • The Sentries actually might be based off of the Sentry Turret from Valve's Team Fortress 2.
  • The BMC Mobile, Sentry is missing the "OnlyTargetPlacementLocations": true line of code it has in BTD5 and Battles, causing the sentries to spawn anywhere, including on the track, water and unplaceable spots.
  • When a sentry runs out of time, it explodes. The explosion is visually the same as the Splodey Darts one from BTD5.
  • If the player sells the engineer, his sentries would be gone instantly after selling the engineer (Mobile only).
Community content is available under CC-BY-SA unless otherwise noted.