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For the BTD5 tower, see Monkey Engineer.

Holds a trusty nail-gun to pop the Bloons. Can upgrade to create its own nail-shooting sentry turrets.
~ In-game Description

The Engineer Monkey is a Support-class tower that arrived in the 12.0 update of Bloons TD 6 on September 17, 2019. The tower received a name-change from Monkey Engineer, but retains its role from the Bloons TD 5 Generation, with significant additions.

It was first teased on social media (specifically on Twitter and Reddit) just before Version 12.0 was released several days later. On Reddit, it was teased here, on September 15th 2019.

Engineer Monkey wields a nailgun that shoots nails at bloons. He shoots one nail every 0.70s, and each nail pierces up to 3 bloons. Engineer starts with a base range of 40. The main power of the Engineer comes from contraptions and other applications of mechanical engineering. He can be upgraded to produce sentries of various types, Cleansing Foam, and Bloon Traps. He can also be upgraded to boost other towers by the use of a power wrench, amplifying the attack speed of its targets with Overclock.

It costs $380 on Easy, $450 on Medium, $485 on Hard, and $540 on Impoppable.


The Engineer Monkey features his yellow hard hat as he did back as Monkey Engineer in BTD5. His nailgun is smaller than his BTD5 counterpart, and is colored yellow. Because of the nailgun's smaller size, he only needs to hold it with just his right hand instead of with both hands.

Path 1 focuses on damage and deployment of Sentries. It allows Engineer to build secondary towers as decent backup defense, with further upgrades increasing their effectivity. Sentry Expert allowing the build of Specialized sentries to add versatile support against certain bloons. Its 5th tier Sentry Champion allows building of very powerful unstable sentries that before disappearing explode (Dealing huge damage to nearby bloons.

Path 2 focuses on support. Larger service area allows the engineer to shoot from a longer range, while Deconstruction improves MOAB and Fortified damage. Cleansing Foam removes Camo and Regrow properties off bloons, Overclock allows monkey engineer to temporarily increase attack speed of targeted tower by 66%. (Duration varies by tier of tower) (For banana farms it increases income and for villages it increases influence radius)., Ultraboost adds permanent 6,6% to overclocked tower that stacks up to 10x for maximum of 66% permanent increase.

Path 3 focuses on a combination between stalling and cleanup. its Pin upgrade allows monkey engineer to pin any Non Moab class, ceramic, lead. Temporarily stopping it from progressing for short time. Bloon Trap allows any non-moab class bloon to be trapped until full which can be manually collected (with some bonus cash). Its 5th tier XXXL Bloon Trap significantly increases the capacity and can also trap Moab class bloons (Excluding B.A.D).

Target Priorities

The base Engineer Monkey uses the standard target priorities: First, Last, Close, Strong. Most of the Engineer's alternate attacks do not integrate its own target priority.

  • First: Targets the first bloon visible within its range.
  • Last: Targets the last bloon visible within its range.
  • Close: Targets the closest bloon visible within its range.
  • Strong: Targets the strongest bloon visible within its range.

All of the Engineer Monkey's alternate attacks have a different set of target priorities:

  • Sentry spawning tends to follow based on the Engineer's perception of where bloons will most likely move around. Hence, sentry spawning depends slightly on RNG but is not entirely RNG-based. Sentry spawns also attempt to only target unoccupied areas of the map. If there are many towers within range of the Engineer blocking potential land spots, the Engineer will spawn sentries between the space of other towers.
  • Sentries themselves start with the target priority of the main Engineer at the time of spawning. Individual sentries must have their target priority changed manually from their current target priority.
  • Cleansing Foam attacks are spawned randomly within the Engineer's range.
  • Overclock and Ultraboost abilities require user input to pick the tower to boost.
  • Bloon Traps and XXXL Traps are always placed on Close regardless of the Engineer's own target priority.


Path 1

Sentry Gun
COST: $425 / $500 / $540 / $600

Description: Creates temporary sentry guns and deploys them nearby.
Effect: Produces temporary sentry guns onto nearby land. Sentry Guns pop up to 2 bloons per shot, lasts for up to 25 seconds before disappearing, and can be improved with crosspathing.
Faster Engineering
COST: $340 / $400 / $430 / $480

Description: Increased efficiency allows the Engineer to produce sentries, foam and bloon traps more often.
Effect: Produces all contraptions +66% faster (0.60x attack cooldown)
COST: $445 / $525 / $565 / $630
UNLOCK: 1,850 XP

Description: Increases sentry gun and Engineer attack speed.
Effect: Nailgun and newly deployed sentry guns attack +66% faster (0.60x attack cooldown).
Sentry Expert
COST: $2,125 / $2,500 / $2,700 / $3,000
UNLOCK: 9,500 XP

Description: Sentries deploy with different damage types based on your need.
Effect: Replaces all new sentries with four specialized sentries: Crushing, Boom, Cold, and Energy. Adjusts deployment of these four sentry types depending on bloons on the screen.
Sentry Champion
COST: $27,200 / $32,000 / $34,560 / $38,400
UNLOCK: 30,000 XP

Description: Creates super-powerful but highly unstable sentries.
Effect: Replaces all new sentries with powerful plasma sentries. Expired sentries now explode a wave of plasma instead of simply disappearing.

Path 2

Larger Service Area
COST: $215 / $250 / $270 / $300

Description: Shoots further and deploys sentries in a much larger area. Sentries have longer range
Effect: Engineer gains +20 range and any newly deployed sentries gain +4 range.
COST: $295 / $350 / $380 / $420

Description: Nail gun and Sentry shots do extra damage to MOAB-class and Fortified Bloons.
Effect: Nailgun and newly deployed sentries deal +1 damage to MOAB-class and +1 damage to Fortified.
Cleansing Foam
COST: $680 / $800 / $865 / $960
UNLOCK: 2,000 XP

Description: Sprays foam that removes Camo and Regrow and pops Lead Bloons.
Effect: Sprays foam anywhere on nearby bloon track. Bloons that hit the foam will permanently lose Camo and Regrow properties. Deals 1 damage to any passing Lead Bloons.
COST: $11,475 / $13,500 / $14,580 / $16,200
UNLOCK: 14,000 XP

Description: Overclock Ability: Target another tower to supercharge its attack speed for a short time.
Effect: Ability temporarily increases attack speed of tower, range of a Monkey Village, or production of a Banana Farm. Ability duration depends on tier of the selected tower. Main attack gains more pierce.
COST: $102,000 / $120,000 / $129,600 / $144,000
UNLOCK: 35,000 XP

Description: Ultra-Overclocked Monkeys gain a small but permanent boost every time they are overclocked.
Effect: Ability now permanently applies a 10th of the bonus of Overclock onto the selected tower.

Path 3

Oversize Nails
COST: $380 / $450 / $485 / $540

Description: Massivization allows nails to pop 8 Bloons at once, including frozen. Also increases Foam and Sentry effectiveness.
Effect: Nailgun pierce increases from 3 to 8. Pierce of sentries and foam improves.
COST: $185 / $220 / $235 / $265

Description: Pins Bloons in place for a short time when struck.
Effect: Nailgun nails pin any non-blimp bloon other than Leads and Ceramics. Can give Pin effect to sentries with Sprockets or above.
Double Gun
COST: $425 / $500 / $540 / $600
UNLOCK: 1,800 XP

Description: Two guns for twice the fun.
Effect: Double nailgun attack speed.
Bloon Trap
COST: $2,975 / $3,500 / $3,780 / $4,200
UNLOCK: 11,000 XP

Description: Bloon trap captures Bloons until full. Select to empty full trap for cash.
Effect: Produces a Bloon Trap that captures up to 500 RBE worth of non-blimp bloons. Manually collecting the full trap earns a bonus +100% income compared to popping them normally.
COST: $45,900 / $54,000 / $58,320 / $64,800
UNLOCK: 40,000 XP

Description: Huge Bloon traps can trap some of the largest Bloons in them...
Effect: Produces a huge XXXL Bloon Trap that captures up to 10,000 RBE worth of almost any bloon including all non-BAD blimps. Manually collecting the full trap earns a bonus +100% compared to popping them normally.

Total Costs

Note: these numbers refer to Medium Mode.

Tier 1 Tier 2 Tier 3 Tier 4 Tier 5
Path 1
Base Cost 500 400 525 2,500 32,000
Cumulative Cost 700 1,100 1,625 4,125 36,125
Base Sell 350 630 2,185 2,748 25,138
Cumulative Sell 665 945 2,500 3,063 25,453
Path 2
Base Cost 250 350 800 13,500 120,000
Cumulative Cost 700 1,050 1,850 15,350 135,350
Base Sell 175 420 980 10,430 94,430
Cumulative Sell 490 735 1,295 10,745 94,745
Path 3
Base Cost 450 220 500 3,500 54,000
Cumulative Cost 900 1,120 1,620 5,120 59,120
Base Sell 315 469 819 3,269 41,069
Cumulative Sell 630 784 1,134 3,584 41,384


Main article: Engineer Monkey/Strategies

Monkey Knowledge

This section displays Monkey Knowledge that has effects on Engineer Monkey and its changes when compared to one by default:

  • VigilantSentriesIcon.png Vigilant Sentries: Sentry Turrets last longer than normal.
  • MonkeyEducationIcon.png Monkey Education: All Monkeys XP earn rate increased by 8%.
  • ThickerFoamsIcon.png Thicker Foams: Cleansing Foam can hit 3 more Bloons before expiring.
    • Increases pierce of each base cleansing foam blob from 10 to 13.
    • With Oversize Nails, cleansing foam blobs increase pierce from 15 to 18.
  • BetterSellDealsIcon.png Better Sell Deals: All Monkeys sell for 5% more.
    • Increases sell potency from 70% to 75%.
  • BigTrapsIcon.png Big Traps: Bloon Trap can hold up to 530 RBE.
  • VeteranMonkeyTrainingIcon.png Veteran Monkey Training: All Monkeys reload time reduced by 3%.
  • GlobalAbilityCooldownsIcon.png Global Ability Cooldowns: All Ability cooldowns for all Monkeys reduced by 3%.


It was teased a few days earlier on Reddit, on September 15th 2019. Other teasers posted on Ninja Kiwi's Twitter on here and here tease the Engineer Monkey.

Version History

12.0 (compared to BTD5 Mobile)
  • Buff Engineer added to the game.
  • Buff Sentries no longer consume Footprint.
  • Nerf Sentries now have 1 pierce per nail instead of 2.
  • Nerf Bloon Trap now has decreased income gain dependent on round income penalty, similar to BTD5 Deluxe and Monkey City Flash.
  • Change Several changes in path placement of the upgrades. Sprockets is now a Path 1 Tier 3 upgrade, Cleansing Foam is now a Path 2 Tier 3 upgrade, Bloon Trap is now a Path 3 Tier 4 upgrade, and none of these upgrades may be bought on the same tower.
  • Nerf Cleansing Foam now has less pierce (30 → 10).
  • Nerf Bloon Trap now has a longer placement cooldown whenever trap is destroyed (~1.0s → ~5.0s) and is limited to 1 per round. The XXXL Trap upgrade can still place unlimited Traps.
  • Change For each round when that trap still stays on the map, Engineer will store another Bloon Trap on itself, and the effect stacks with the number of rounds passing per Engineer. When all Bloon Traps are used during a later round, Engineer won't spawn another until next round.
  • Buff Larger Service Area upgrade costs less ($650 → $550).
  • Buff Sprockets upgrade costs less ($550 → $525).
  • Nerf Overclock ability now has an initial cooldown of 15 seconds before usage when the upgrade is first purchased.
  • Buff Overclock ability can now target itself.
  • Nerf Overclock ability duration decreases to 45s for towers valued between $15,000-$39,999, and to 30s for towers valued $40,000+.
  • Buff Bloon Trap now traps more RBE (250 → 500).

Balance Changes

  • Buff 4-x-x Sentry Expert Energy Sentry damage increased from 1 to 2.
  • Buff 4-x-x Sentry Expert Bomb Sentry damage increased from 1 to 2.
  • Buff 4-x-x Sentry Expert Bomb Sentry attack rate increased from 1.5s to 1.4s.
  • Buff 4-x-x Sentry Expert Spiked Ball attack rate increased from 1.25s to 1s.
  • Buff 5-x-x Sentry Champion explosion damage increased from 70 to 85.
  • Buff 5-x-x Sentry Champion manually sold sentries will now explode with no animation delay.
  • Nerf x-4-x Overclock reload time reduction lowered from -50% to -40% (farms remain 2x speed and villages still receive 25% range).
  • Nerf x-5-x Ultraboost permanent reload time reduction lowered to -4% for a maximum of -40%.
  • Buff 1-x-x Standard Sentry pierce increased from 1 to 2.
  • Buff 4-x-x Sentry Expert price reduced from $2,800 to $2,500.
  • Buff 5-x-x Paragon Sentry pierce increased from 3 to 5.
  • Buff Sentry Expert's Boom Sentry blast radius increased (12 → 18).
  • Buff Sentry Expert's Boom Sentry attack speed increased (1.4 → 1.3).
Continuing our efforts to make crosspaths meaningful and fit the style of each path more solidly, a number of crosspath and base level balance shifts were made.
~ Ninja Kiwi
  • Buff 1-1-0 Larger Service Area now applies 1/5th the range (+4) to any sentries produced.
  • Buff 1-0-1 Oversize Nails allows sentries produced by this tower to pop frozen Bloons.
  • Buff 1-0-1 Oversize Nails now increases sentry pierce by +1.
  • Buff 2-x-x Faster Engineering sentry deployment rate increased further from 7s to 6s.
  • Buff 4-x-x Sentry Expert base range of all sentries increased from 45 to 50.
  • Buff 5-x-x Sentry Champion explosion radius now scales with sentry radius.
  • Buff x-4-x Engineer Overclock ability active buff state will now be included in save files.
  • Change x-5-x Ultraboost will now apply 'perma' stacks upon ability activation rather than expiration.
Engineer is quite poor in terms of crosspathing, so we've added crosspath benefits to Cleansing Foam as well as an overall price buff to the middle crosspathing upgrades. We've been somewhat unhappy with how Overclock sits in the game for a while and especially how the interaction with tower cost causes levelling heroes naturally to be far advantageous over purchasing upgrades when this is already underused. We have made an attempt to rework these issues by moving it to a tier-based system rather than price.
~ Ninja Kiwi
  • Buff 5-x-x Sentry Champion explosion damage increased from 85 → 100.
  • Buff 0-1-0 Larger Service Area price reduced from $550 → 250.
  • Nerf 0-2-0 Deconstruction price increased from $200 → 350.
  • Buff 2-3-0 Faster Engineering increases Cleansing Foam rate 2 → 1.2.
  • Buff 0-3-1 Oversize Nails increases Cleansing Foam pierce 10 → 15.
  • Nerf Banana Farms give 66.7% more income instead of 100%.
  • Change Overclock ability is reworked to operate on a tier basis rather than a cost basis:
    • Nerf T5 duration is now always 30 seconds.
    • Nerf T4 duration is now always 45 seconds.
    • Change T3 duration is now always 60 seconds.
    • Buff T2 duration is now always 75 seconds.
    • Buff T1 duration is now always 90 seconds.
    • Buff T0 duration is now always 105 seconds.
    • Nerf Hero duration is based on X/4, where X is level, starting from Level 0 (i.e. Level 1-3 is T0, Level 4-7 is T1, Level 8-11 is T2, Level 12-15 is T3, Level 16-19 is T4, Level 20 is T5).
Bloon Trap can now be used more effectively.
~ Ninja Kiwi
  • Buff If all of an Engineer's stockpiled Bloon Traps are used up, Bloon Trap now has a cooldown of 20s instead of only capping at one new trap per round. Bloon Trap cooldown now starts upon once it spawns rather than once it is collected. Does not affect the existing 2.8s cooldown while refreshing a trap.
    • Note: Bloon Trap is still limited to 1 at a time per Engineer.
  • Buff 2-0-4 Faster Engineering reduces the bonus Bloon Trap cooldown from 20s → 16s. Does not affect the existing 2.8s cooldown per trap.
  • Nerf XXXL Trap will cooldown at 1/6th of the rate of the standard Bloon Trap.
    • XXXL Trap cooldown is now increased from 2.8s to 3.33s.
    • Note: Bloon Trap's minimum deploy time of 2.8s will override 205 Trap's 2.7s cooldown.
Sentry Expert has underperformed for a while, so we're adding solid crosspathing buffs to all Sentries. We've added a pierce & speed boost to Overclock's base upgrade and lowered the cooldown on Ultraboost to allow the possibility for it to achieve full uptime as a freeplay tower and get its stacks up faster. Finally last update we allowed Bloontrap to be set multiple times per round, and we wanted to stay on the side of caution with a long cooldown, but after seeing it in the wild we are happy to make this cooldown faster.
~ Ninja Kiwi
  • Buff Deconstruction now grants all spawned Sentries with +1 damage to MOAB-class and +1 damage to Fortified.
  • Buff 4-0-1 Sentry expert now increases sentry pierce by roughly 25% instead of +1.
  • Buff 4-0-1 Crushing Sentry pierce increased (22 → 28).
  • Buff 4-0-1 Bomb Sentry pierce increased (30 → 38).
  • Buff 4-0-1 Cold Sentry pierce increased (15 → 19).
  • Buff Overclock's main attack's projecitle speed increased (375 → 750).
  • Buff Overclock's main attack's pierce increased (3 → 15).
  • Buff Ultraboost's main attack's pierce increased (3 → 30).
  • Buff Ultraboost ability cooldown reduced (45s → 35s).
  • Buff [undocumented] Ultraboost ability initial cooldown reduced (15s → 5s).
  • Buff Base Bloon Trap rate increased (20s → 15s).
  • Buff 2-0-4 Bloon Trap rate bonus increased (0.8x → 0.6x, or +25% → +66%).
Engineer XXL [sic] trap is extremely powerful but too expensive for backline dps and we'd like to see it more viable for frontline engineer builds.
~ Ninja Kiwi
  • Buff XXXL Trap price reduced from $60,000 → $54,000.
It has been clear that Sentry Expert needs changes, with Cryo sentries the only ones that really seemed valuable and purposeful, so all of the others have been improved to fit their role. Additionally higher tier sentries will now be able to benefit from the xx2 Pin crosspath, so enjoy some laser pins. Bloon Trap redeployment rate has also been buffed a fair amount.
~ Ninja Kiwi
  • Buff 4-x-0 Laser Sentry gains pierce increased from 4 to 8.
  • Buff 4-0-1 Laser Sentry gains pierce increased from 5 to 10.
  • Buff Ball Sentry damage increased from 1 to 2.
  • Nerf Ball Sentry attack cooldown increased from 1s to 1.1s.
  • Buff Boom Sentry attack cooldown reduced from 1.3s to 0.9s.
  • Buff 3-0-2 Sentries & Higher will now also be able to Pin Bloons they pop.
  • Buff Bloon Trap redeployment cooldown reduced from 15s to 12s.
  • Buff [undocumented] XXXL Trap minimum cooldown is now reworked to scale properly with attack speed buffs. Therefore, new minimum cooldown is reduced from a fixed 2.8s to a flexible 2.0s.
  • Change Sentry Paragon renamed to Sentry Champion to differentiate it from Paragons.

Bug Fixes and General Changes

  • Change Larger Service Area now increases Sentry deploy range (Arguably a nerf).
  • Nerf Fixed a bug where unupgraded Engineers gained the Deconstruction buff.
  • Nerf Fixed a bug where Bloon Trap granted camo detection and lead poppage to sentries.
  • Nerf Engineer Monkey Knowledge prerequisites have been fixed.
  • Nerf x-x-4 Bloon Trap no longer collects for reduced cash when under a Monkey Town.
  • Change Acquisition range is correctly blocked by obstacles on High Finance.
  • Change xx4 Bloon Trap should collect for the correct amounts after resuming save.
  • Nerf x-x-4 Bloon Trap will no longer allow bonus traps to be placed when reloading save.
  • Buff x-4-x Overclock can now be targeted by Techbot.
  • Buff x-5-x Ultraboost will no longer trigger while techbotted if no target was previously set.
  • Nerf 1-x-x Sentry Gun sentries no longer gain camo detection when a save is loaded.
  • Buff 1-x-x Sentry Gun sentry placement will no longer ignore 'smart placement' if the Engineer cannot reach the track.
  • Nerf 5-x-x Sentry Champion should no longer cause a secondary explosion when manually sold.
  • Buff x-x-4 Bloon Trap, resolved collection sometimes displaying less cash than it should.
  • Buff 5xx Paragon Sentry "can always sell" property on sentries has been fixed to work in challenges as well.
  • Changex-2-x Deconstruction description has been updated with more detail.
  • Changex-4-x Overclock description has been updated with more detail.
  • Changex-5-x Ultraboost description has been updated with more detail.
  • Changex-x-1 Oversize Nails description has been updated with more detail.


Official artwork




  • The tower itself is a reference to the Engineer class in Team Fortress 2.
  • The engineer appears as the icon for Support towers in the Monkey Knowledge screen in BTD6 meaning it was most likely intended to be a launch tower but was not finished in the time frame they wanted to release the game in.
    • The engineer was eventually released in the 12.0 update with a different artwork than the Monkey Knowledge icon.
  • In BTD6, this was the only tower where all of its tier 2 upgrades cost less than its tier 1 upgrades.
    • This is no longer true as of the 21.0 update, since Deconstruction now costs more than Larger Service Area.
  • Sentries inside the range of a Monkey Village will have the effects of it, regardless of whether the Engineer that spawned them is inside the range of the Monkey Village.
  • Overclock is the only Tier 4 Engineer Monkey to lack any form of eyewear. Sentry Expert wears experimental goggles, while Bloon Trap wears a pair of yellow tinted sunglasses.
  • Sentry Champion formerly had the name 'Sentry Paragon', but was renamed to avoid confusion with the later added Paragons.
  • According to Ninja Kiwi's October 29th 2021 blog, Engineer Monkeys trying to hit Paragon Sentries (referred as "sentry champion [sic]" in the blog) to speed them up causes these sentries to explode. Since then, Engineer Monkeys never hit their sentries ever again. Because hitting sentries to speed them up is a Team Fortress 2 reference, it can be implied that Engineer Monkeys use wrenches to hit their sentries.