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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. It allows the Engineer to create 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. If there are certain bloon types detected within the Engineer's radius, he will skip certain sentry types in the sentry-spawning cycle.

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

Types of Sentries[]

There are four types of sentry that the Engineer may produce. All sentry types despawn after 25 seconds (30 with 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, while Pin also 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. These balls deal 2 damage plus another +1 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.

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

If Ceramic Bloons are present it will be the only sentry that can spawn.

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, roughly 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.

  • Damage Type: Explosion Type (pops White, Purple, Lead, Frozen) Explosion
  • Damage: 2
  • Pierce: 30 (38 with 4-0-1)
  • Attack Speed: 1.11x (every 0.9s)

If Ceramic, Black, Zebra or DDT Bloons are present it will not spawn.

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 roughly ~15. Cannot affect Whites, Zebras, or Leads. The cold sentry roughly acts like a weaker 1-0-3 Cryo Cannon.

  • Damage Type: Cold Type (pops Black, Purple) Cold
  • Damage: 1
  • Pierce: 15 (19 with 4-0-1)
  • Attack Speed: 0.67x (every 1.5s)

If Ceramic Bloons are present it will not spawn.

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.

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

If Ceramic Bloons are present it will not spawn.



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. As a whole, Sentry Expert is decent but very unreliable due to its awkward sentry-spawning algorithms.

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. Because of this, it can become a hassle for the Sentry Expert to produce the right combination of special sentries that will become the most favorable to your given situation.


  • Just like Laser Blasts, the Energy Sentry can't pop Lead or Purple Bloons.
  • The Crushing Sentry behaves similar to the Juggernaut upgrade, minus that 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.
  • When choosing crosspaths, be sure to think about which effect would be more useful. 4-2-0 improves damage to MOAB-class and Fortified, but these sentries attack slowly in exchange for more pierce, which may not be the best choice versus MOAB-class and Fortified than the preceding Sprockets upgrade due to the slow attack speed of most of its sentries. 4-0-2 is more effective in terms of support, as it allows the high pierce Laser and Crushing Sentries to pin Bloons in place.
  • Adding a small group of Sentry Experts can handle Round 63 adequately, as well as deal adequate stalling on late-game rounds via the Cold Sentries.


Bug Reports[]

[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]

Bug Fixes[]


Version History[]

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 1.5 → 1.4.
  • Buff Crushing Sentry attack rate increased from 1.25 → 1.
  • Buff Sentry Expert price reduced from $2800 to $2500.
  • Buff 4xx Sentry Expert's Boom Sentry blast radius increased 12 → 18.
  • Buff 4xx 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


When a sentry is placed:

  • [gadget placement]
  • [gadget placement]


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 his pencil in his 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.