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Sentries deploy with different damage types based on your need.
~ In-game description

Sentry Expert is the 4th upgrade of Path 1 for the Engineer Monkey in Bloons TD 6. In exchange for the Engineer's normal sentries, the Engineer creates specific sentries that specialize in popping specific bloon types. There are four unique sentry types that may spawn from the Sentry Expert, each with unique attacks: Crushing, Boom, Cold, and Energy. The Engineer may produce specific types depending on what bloons are present. There is no manual control over what sentry types may or may not spawn.

When no special Bloon types are present with the Sentry Expert's range, the Sentry Expert cycles between Crushing, Boom, Cold and Energy Sentries in that order. The Engineer evaluates what sentries to spawn depending on the current types of bloons on screen, and it can skip certain sentry types in the sentry-spawning cycle in order to smartly choose the best sentries in particular scenarios.

This upgrade costs $2,125 on Easy, $2,500 on Medium, $2,700 on Hard, and $3,000 on Impoppable.



Sentry expert all sentries

Sentry Expert with all four sentry types active

Sentry Expert gives the Engineer a pair of green goggles with a red laser sight above the left lens, as well as a black backpack. Their combination wrench now sports a green handle. Instead of ordinary green sentries, the Sentry Expert produces four different types of sentry that shoot projectiles of different damage types.

Sentry Expert changes its normal Sprockets Sentries with four specialized sentries: Crushing, Boom, Cold, and Energy. Sentry Expert attempts to choose the best sentries for a given scenario. The four sentry types it deploys are colored, respectively, red (Crushing), black (Boom), ice blue (Cold), and yellow (Energy), all with a green stripe. Each of these sentries act differently:

  • Crushing Sentries shoot spiked balls that rebound obstacles and deal extra Ceramic damage
  • Boom Sentries shoot explosive bombs that deal high damage
  • Cold Sentries shoot snowballs that freeze bloons
  • Energy Sentries shoot high-damage red lasers that deal extra MOAB-class damage

The Crushing sentry's projectiles are black balls with white spikes, the Boom sentry's projectiles are large black spherical bombs, the Cold sentry's projectiles are small ice balls, and the Energy sentry's projectiles are thin red laser blasts.

The artwork shows the Engineer standing in a confident pose with arms outstretched, the four sentry types arrayed before it.

Crosspathing with path 2 gives the Engineer a yellow pencil sticking out of its backpack. Crosspathing with path 3 gives the Engineer's helmet a white front, and its nail gun a light grey extension to the barrel before swapping it out for an entirely grey helmet and orange and black gloves.

For further information about the sentries and its spawning algorithms, refer to the #Types of Sentries section.

Targeting Priorities[]

Sentry Expert targets bloons using the standard targeting priorities. Its main attack shoots straight movement projectiles.

  • 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.

Sentry Expert, like all Engineers with Path 1 upgrades, can deploy sub-towers called sentries, targeting them such that they prioritize unoccupied land. Newly spawned sentries inherit the parent Engineer's targeting priorities, and also shoot straight movement projectiles. Targeting priority for all sentries use standard targeting priorities (First, Last, Close, Strong) and can be changed at any time.


Updated as of Version 43.0. Listed prices exclude external buffs, any sacrifices, and Monkey Knowledge.
[Module | Data] Purchase Sell
Costs Easy Medium Hard Impoppable Easy Medium Hard Impoppable
Upgrade (4-X-X) $2,125 $2,500 $2,700 $3,000 -
Total (4-0-0) $3,675 $4,325 $4,670 $5,190 $2,573 $3,028 $3,269 $3,633
Crosspath (4-1-0) $3,885 $4,575 $4,940 $5,490 $2,720 $3,203 $3,458 $3,843
Crosspath (4-2-0) $4,180 $4,925 $5,320 $5,910 $2,926 $3,448 $3,724 $4,137
Crosspath (4-0-1) $4,055 $4,775 $5,155 $5,730 $2,839 $3,343 $3,609 $4,011
Crosspath (4-0-2) $4,240 $4,995 $5,390 $5,995 $2,968 $3,497 $3,773 $4,197

This upgrade's prices (or included crosspath's prices) are affected by the following MK: BetterSellDealsIcon Better Sell Deals

Types of Sentries[]

There are four types of sentry that the Engineer may produce. All sentry types despawn after 25 seconds (30 with VigilantSentriesIcon Vigilant Sentries MK). All Sentries have 50 range and gain +4 range with Larger Service Area and +25% pierce rounded up with Oversize Nails. Deconstruction allows sentries to deal +1 MOAB-class and +1 Fortified damage. Pin gives the sentries with a pinning effect, even though none of their projectiles are made of nails.

Crushing Sentry[]


Crushing Sentry

The crushing sentry appears red and shoots spiked balls reminiscent of the Dart Monkey's top path upgrades, having both the bounciness and the same amount of pierce as Spike-o-pult but roughly the damage of Juggernaut. These balls deal 2 damage plus another +4 damage to Ceramic Bloons and are able to bounce off obstacles. It has a very slightly greater attack speed than Spike-o-pult and deals more damage to all bloons it can hit. Like Spike-o-pult, Crushing Sentry cannot pop Lead Bloons without support but always affects Frozen Bloons.

Crushing Sentry packs a lot of pierce and Ceramic damage, being the Ceramic destruction specialist. Sentry Expert uses Crushing Sentries in situations where large masses of bloons must be cleaned up, especially against Ceramics, but will swap it out for Boom Sentry if there are Leads. It also appears during the late-game to clean up Super Ceramics, often in combination with Cold Sentry.

  • Damage Type: Shatter Type (pops Black, White, Purple, Frozen) Shatter
  • Damage: 2 (+4 to Ceramics)
  • Pierce: 22 (28 with 4-0-1)
  • Attack Speed: 0.91x (every 1.1s)

Boom Sentry[]


Boom Sentry

The boom sentry appears black and is slightly larger than other sentries and shoots bombs reminiscent of the Bomb Shooter. Bombs pop bloons in a small radius (18 radius, slightly less than the Ice Monkey's base range) and can pop Frozen Bloons and Lead Bloons. The boom sentry roughly acts like a 2-1-0 Bomb Shooter with the attack speed of a Missile Launcher and dealing twice the damage of Heavy Bombs.

Boom Sentry specializes in densely packed group damage and can pop Leads but not Black or Zebras. It is typically accompanied by Crushing Sentry to deal with Blacks and Zebras. Boom Sentry is expected to appear more often in pre-Round 81 rounds due to its high damage to regular bloons. Boom Sentry always appears in the cycle of special sentries whenever there are Leads, as all other sentry types cannot deal with them naturally.

  • Damage Type: Explosion Type (pops White, Purple, Lead, Frozen) Explosion
  • Damage: 4
  • Pierce: 30 (38 with 4-0-1)
  • Attack Speed: 1.11x (every 0.9s)

Cold Sentry[]


Cold Sentry

The cold sentry appears blue and shoots snowballs reminiscent of the Ice Monkey's Cryo Cannon upgrade. Affected bloons are frozen for 1.5s and also gain the Permafrost effect. Snowballs pop and freeze bloons in a small radius, with a blast radius of 18. Cannot affect Whites, Zebras, or Leads. The cold sentry roughly acts like a weaker 1-0-3 Cryo Cannon, although Cold Sentry is unable to refresh a freeze effect. It can damage Frozen Bloons but not re-freeze them (which is noticeable when dealing damage to non-Ceramics but unable to re-freeze a Ceramic regardless of speed buffs, respectively).

Despite its name, it actually uses the Glacier Type (pops Black, Purple, Frozen) Glacier damage type, not Cold Type (pops Black, Purple) Cold, although the Freeze (standard bloons, unspecified alt variants) Freeze status effect is separately a different effect with damage type Cold Type (pops Black, Purple) Cold due to being unable to actually re-freeze bloons. Since Version 25.0, which revamped the classification for freezing effects with standard damage types, a discrepancy between the Cold Type (pops Black, Purple) Cold damage type and the Freeze (standard bloons, unspecified alt variants) Freeze status effect having inconsistent outcomes on Frozen Bloons still remain for Cold Sentry.

Cold Sentry specializes in freezing bloons in place. Sentry Expert tends to use them for freezing Pinks in place, reducing their progress. Cold Sentry is often used combination with Crushing and Boom Sentries, or Crushing and Energy Sentries if there are lots of MOAB-class bloons.

Snowball (damage)

  • Damage Type: Glacier Type (pops Black, Purple, Frozen) Glacier
  • Damage: 1
  • Pierce: 25 (31 with 4-0-1)
  • Attack Speed: 0.67x (every 1.5s)

Snowball (freeze)

  • Damage Type: Cold Type (pops Black, Purple) Cold
  • Damage: 0
  • Pierce: 25 (31 with 4-0-1)

Energy Sentry[]


Energy Sentry

The energy sentry appears yellow and shoots red laser blasts reminiscent of the Dartling Gunner's Laser Cannon upgrade. Energy Sentry has the highest attack speed but the lowest pierce of any sentry. Laser blasts can pop frozen bloons but cannot pop Leads or Purples. It attacks a lot slower than any laser-related upgrade (i.e. Laser Blasts and Laser Cannon, the only two other upgrades with true lasers) but deals 2 damage and 8 pierce (or 10 with 4-0-1) each, plus another +2 damage to MOAB-class.

Energy Sentry deals the most MOAB-class damage of the specialized sentries. As it does optimized damage to MOAB-class bloons, Energy Sentries are more likely to be spawned if there are lots of MOAB-class bloons. Energy Sentries are generally swapped out if there are lots of Purples, often in favor of Cold and Crushing Sentries. If only Blue MOABs are on screen, the Sentry Expert may somewhat favor adding a single Crushing Sentry in its cycle of sentries. Energy Sentries may also be favoured in the presence of Rainbow Bloons.

  • Damage Type: Energy Type (pops Black, White, Frozen) Energy
  • Damage: 2 (4 versus MOAB-class)
  • Pierce: 8 (10 with 4-0-1)
  • Attack Speed: 1.75x (every 0.57s)


Certain types of Bloons will change Sentry Expert's priorities


  • Bloon Type - The Bloon present on screen
  • Will Prioritize Spawning - Place all the sentries listed
  • Will Avoid Spawning - Place all the sentries except the listed one(s)
  • Will Occasionally Spawn - May Spawn the Sentry listed (Not Often)
Bloon Type Will Prioritize Spawning Will Avoid Spawning Will Occasionally Spawn
Red Bloon None None None
Blue Bloon None None None
Green Bloon None None None
Yellow Bloon None None None
Pink Bloon Cold Sentry None None
Black Bloon None Boom Sentry None
Purple Bloon Crushing Sentry None None
White Bloon None Cold Sentry None
Lead Bloon Boom Sentry None None
Zebra Bloon Energy Sentry None None
Rainbow Bloon Energy Sentry None Cold Sentry

(Once Pink Bloons Appear)

Ceramic Bloon Crushing Sentry None Cold Sentry

(Once Pink Bloons Appear)

M.O.A.B. Energy Sentry None Cold Sentry

(Once Pink Bloons Appear) Crushing Sentry (Once Ceramic Bloons Appear)

B.F.B. Energy Sentry None Cold Sentry

(Once Pink Bloons Appear) Crushing Sentry (Once Ceramic Bloons Appear)

Z.O.M.G. Energy Sentry None Cold Sentry

(Once Pink Bloons Appear) Crushing Sentry (Once Ceramic Bloons Appear)

D.D.T. Cannot Detect Cannot Detect None
D.D.T. (Not Camo) Energy Sentry None None (Cannot Pop outer Layer)
B.A.D. Energy Sentry None Cold Sentry

(Once Pink Bloons Appear) Crushing Sentry (Once Ceramic Bloons Appear)



Sentry Expert is a situational upgrade for creating sentries designed for various tasks that lower-tier Engineers are unable to achieve alone. On one hand, Sentry Expert's various sentry types contribute as a supportative defensive role quite well, thanks to a combination of utility and extra damage through its specialized sentries, which pack with greater pierce and damage at the cost of slower attack speed overall. On the other hand, Sentry Expert can fail to be useful if it cannot properly produce the right sentries for the job. In most situations, Sentry Expert's sentry-spawning algorithms should be fairly reliable overall, adapting distribution of sentry types to accommodate various bloon spawn cases.

Certain sentry types can be very useful in different situations:

  • Energy Sentries are powerful at damaging MOAB-class bloons and popping up bloons effectively. Its high pierce and moderately quick attack speed helps it become effective for both usages.
  • Cold Sentries are helpful at stalling Ceramics, especially Super Ceramics, more effective compared to Pin in that niche. However, Cold Sentries can be more harmful than good in situations where most of the defense consists of sharp-based projectiles.
  • Crushing Sentries have immense popping power and can work effectively in locations with poor Line of Sight, as the ball projectiles are able to bounce off walls, similar to Spike-O-Pult.
  • Boom Sentries allow popping of Leads and deals a considerable amount of damage to dense clumps of bloons. They can also deal decent cleanup against weaker clumps of bloons.

However, there is very limited control over the Sentry Expert's production of certain sentry types. The Sentry Expert attempts to produce the right combination of special sentries that are the most favorable to the player's given situation, often producing sentries in a very specific algorithm. Most of the time, this can work very favorably, although there is still an element of getting unfavorable sentry types. To combat this, it is best to pair Sentry Expert with multiple of its own type, as well as using Sentry Expert on top of an already fairly balanced defense. Its strongest value is when utilized as a cost-effective supporting unit rather than a singular defensive option.


  • Sentry Expert does not need crosspaths in order to suffice as a supporting unit, due to its already high pierce and MOAB-class damage, but greatly appreciates from them. 4-2-0 improves damage to MOAB-class and Fortified, although only slightly since Energy Sentries already deal high MOAB-class damage per shot. 4-0-2 is more effective in terms of support during the mid-game, allowing all sentries other than Cold Sentry to pin Bloons in place.
    • While the Sprockets upgrade creates much faster-attacking sentries than Sentry Expert, the Sentry Expert's Energy Sentry deals a lot more MOAB-class damage per shot at equivalent attack speed to the Sprockets sentries.
  • The Crushing Sentry behaves similar to the Juggernaut upgrade, except it cannot damage Lead Bloons. As a result, they can be really useful on maps with many obstacles, such as Cornfield.
  • Similar to totems, sentries cannot be buffed by Alchemists. Buffing the Engineer will not buff the Sentries, but it will improve the production rate of sentries.
  • Adding a small group of Sentry Experts can handle Round 63 adequately, as well as adequately stall late-game rounds via the Cold Sentries.
  • Some rounds are particularly notable for observing the Sentry Expert's sentry-placing algorithm.
    • Round 63 tends to combine Boom and Cold Sentries plus Crushing Sentries, utilising the Boom Sentries to explode Leads and the Cold and Crushing Sentries to stall and break the Ceramics. In combination, Sentry Expert is quite strong against this round.
    • Round 53 mixes large amounts of Camo Pinks in a very spaced out formation plus MOABs. This is a weak spot of pure Sentry Expert spam, as the Pinks just cause far too many Cold Sentries and not bring enough Energy Sentries.
    • Round 40 currently gives a 3-to-1 (?) ratio of Energy Sentries to Crushing Sentries under any circumstance, no matter the health of the MOAB.


  • [version unknown] Even though it cannot affect them, cold sentries will still spawn if there are Lead Bloons. [TBA]
  • [version unknown, possibly related to the above bug] There is a bug/special case where only cold sentries will spawn when there are DDTs present. [TBA]


Version History[]

Balance Changes[]

Sentry Expert had been buffed a considerable amount since it was first released. The Sentry Expert upgrade is more powerful, but still fits only on rather niche situations, mostly due to the nature of specialized sentry spawning.

  • BUFF Energy Sentry damage increased from 1 → 2.
  • BUFF Boom Sentry damage increased from 1 → 2.
  • BUFF Boom Sentry attack rate increased from 1.5s → 1.4s.
  • BUFF Crushing Sentry attack rate increased from 1.25s → 1s.
  • BUFF Sentry Expert price reduced from $2800 to $2500.
  • BUFF Sentry Expert's Boom Sentry blast radius increased 12 → 18.
  • BUFF Sentry Expert's Boom Sentry attack rate increased 1.4s → 1.3s.
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, referring to general Engineer crosspathing benefits
  • BUFF Sentry Expert is now affected by crosspathing from Larger Service Area and Oversize Nails.
  • BUFF Sentry Expert sentries are deployed faster (7s → 6s).
  • BUFF Sentry Expert base sentry range increased (45 → 50).
Sentry Expert has underperformed for a while, so we're adding solid crosspathing buffs to all Sentries. [...]
~ Ninja Kiwi, referring to both Sentry Expert and Engineer's Sentries in general
  • BUFF 4-0-1 Sentry Expert's sentries now gain +25% pierce instead of a straight +1 pierce.
    • 4-0-1 Crushing Sentry pierce increased (22 → 28).
    • 4-0-1 Bomb Sentry pierce increased (30 → 38).
    • 4-0-1 Cold Sentry pierce increased (15 → 19).
  • BUFF Sentry Expert sentries can now benefit from Deconstruction
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. [...]
~ Ninja Kiwi
  • BUFF 4-x-0 Laser Sentry gains pierce increased from 4 → 8
  • BUFF 4-0-1 Laser Sentry gains pierce increased from 5 → 10
  • BUFF Ball Sentry damage increased from 1 → 2
  • NERF Ball Sentry attack cooldown increased from 1s → 1.1s
  • BUFF Boom Sentry attack cooldown reduced from 1.3s → 0.9s
  • BUFF 4-0-2 allows all sentries to apply a Pin effect to bloons that they pop
Sentry Expert really needed some logic fixes to get more of the actual correct sentries placed at the right times, it has also received a number of power buffs across the board to the special sentries at T4 complimenting the new logic reworking. [...]
~ Ninja Kiwi
  • BUFF Sentry Expert logic has been reworked to evaluate threat levels based on Bloon types that it can counter, Bloon types that counter the sentries that it wants to spawn, and which of the current threats on screen is the greatest. Distribution of sentry types should scale based on how much one Bloon threat is more of a threat than another.
    • Sentry Expert formerly used the "if bloon type exists then skip sentry type X, otherwise cycle through the sentries" system.":
      • Crushing: skipped if there are leads (unless black and white are also present), but never skipped if there are ceramics[1]
      • Boom: skipped if there are blacks (unless white and lead are also present)[1]
      • Cold: skipped if there are whites (unless lead and black are also present)[1]
      • Energy: skipped if there are leads (unless black and white are also present), and always skipped if there are purples[1]
  • BUFF Sentry Expert Crush Sentry ceramic damage bonus increased from +1 → +4
  • BUFF Sentry Expert Crush Sentry damage type changed from Sharp → Shatter
  • BUFF Sentry Expert Bomb Sentry damage increased 2 → 4
  • BUFF Sentry Expert Cold Sentry pierce increased from 15 → 25
    • BUFF 4-0-1 Sentry Expert Cold Sentry pierce increased from 19 → 31
  • BUFF Sentry Expert Cold Sentry radius increased from 12 → 18
  • BUFF Sentry Expert Energy Sentry gain moab damage bonus of +2
The MOAB offense of top path lacks for a mainly dps tower that is quite complex to use, T3 Sprockets already feels underwhelming so is seeing a buff here carrying up to T4, and the 320 crosspath will now be able to double dip on the Fortified & MOAB damage bonuses that it gains. [...]
~ Ninja Kiwi
  • BUFF Sentry Expert now benefit Sentry and nail gun attack speed from Sprockets upgrade
  • Note: Ninja Kiwi incorrectly listed the attack speed bonuses as a buff to the production rate; it is actually a buff to the attack speed bonuses granted to the nailgun and sentry nail attacks.
  • BUFF 4-2-0 Sentries' Deconstruction bonuses to Fortified and MOAB-class are no longer mutually exclusive
  • [undocumented, bug] Energy Sentries projectiles ignore line of sight.
Improvements for Sentry Expert AI based on your feedback. [...]
~ Ninja Kiwi
  • BUFF Sentry Expert should react with less Cold Sentries in response to Pink Bloons, and prioritize a more for AoE cleanup (i.e. slightly prioritizes greater for Bomb Sentries)
  • Change Sentry Expert should react with less Energy sentries in response to Blue MOABs, and prioritize a more for Ceramic cleanup (i.e. slightly prioritizes greater for Cold and Crushing Sentries)
[...] Sentry placements are slightly changing to improve their reliability, and will now spawn closer to the track depending on how much range the sentry has. [...]
~ Ninja Kiwi
  • Change Sentries now spawn within 20% of their range to the track

Bug Fixes and General Changes[]

Extra Notes (Version History)[]


When a sentry is placed:


Version 31.0 sentry algorithms[]

Official artwork[]




  • Sentry Expert's upgrade icon features all four types of Sentries that the Sentry Expert can create.
  • When crosspathed with Path 2 crosspaths, Sentry Expert rests its pencil in its carryable toolcase, and the pencil is sticking upwards. Resting a pencil on top of the left ear is what all other Engineers with Path 2 upgrades will do.
  • The Cold Sentry, despite sharing a name with the Cold damage type, is not that damage type, but rather Glacier; this contrasts with the Energy Sentry, which is Energy type.


  1. 1.0 1.1 1.2 1.3 Topper Popology 30.0 Link