Sentry Expert is the Engineer Monkey's path 1 tier 4 upgrade in Bloons TD 6. This upgrade replaces its Sentries with four different types of Sentries that each have a different attack. It can spawn Crushing Sentries, Boom Sentries, Cold Sentries, and Energy Sentries, and it spawns them in that order by default, but it can prioritize and deprioritize each Sentry type depending on what Bloons and Sentries are currently on-screen.
When an Engineer Monkey with Sentry Expert is about to deploy a Sentry, it assigns a score to each type of Sentry based on the types of all Bloons and Sentries on the map at that moment (including Sentries placed by other Sentry Expert Engineer Monkeys), then deploys the Sentry type with the highest score it determined. It only considers the outer layers of Bloons, and does count Camo Bloons even if the Engineer Monkey doesn't have Camo detection. It calculates the score of each Sentry type in the following steps:
The Sentry type's score changes based on the proportions of Bloons currently on the map (the percentage of Bloons of each type out of the total number of Bloons). This works by applying a multiplier to the proportion of each Bloon type it checks for, then adding those to its score modifiers. Each Sentry type has separate score multipliers the Bloon types it checks for, which can be positive or negative; it doesn't check for Bloon types that it doesn't have multipliers for, but they still count towards the total number of Bloons when determining proportions, even Boss Bloons and Frontier Legends-exclusive Bloons.
For example, Boom Sentry applies 1× score to Lead Bloons, -1× score to Black Bloons, and -0.5× score to MOABs; if there are 60 Lead Bloons, 20 Black Bloons, 20 MOABs, and no other Bloons of any type on-screen, Boom Sentry has +0.6 score from Lead Bloons, -0.2 score from Black Bloons, and -0.1 score from MOABs.
In this scenario, if there are also 100 Purple Bloons on-screen, it effectively halves all of these score modifiers, since half of the proportion of Bloons on the map are ones that don't affect the score of Boom Sentry.
Each Sentry type has a special condition that applies another score modifier if met:
Crushing Sentry: +1.5 if there are no Crushing Sentries and at least 15 Ceramics
Boom Sentry: +1.5 if there are fewer Boom Sentries than Cold Sentries and at least 20 Pinks
Cold Sentry: +1.5 if there are fewer Cold Sentries than Boom Sentries and at least 20 Pinks
Energy Sentry: -4 if there are zero Crushing Sentries or zero Cold Sentries, and at least 10 MOABs
The average of all the Sentry type's score modifiers up until this point becomes the base score.
For example, if the Boom Sentry's score modifiers are +0.6 from Leads, -0.2 from Blacks, -0.1 from MOABs, and +1.5 from its special condition, its base score is 0.6. If the special condition is not met, its score modifier doesn't factor into the average, so the base score becomes 0.1 instead.
Finally, the base score lowers by 1% per Sentry of that type that currently exists on the map. For example, if Boom Sentry has a base score of 0.6 and there are already three Boom Sentries on the map, its score becomes 0.582.
If the highest score is tied, it prioritizes Crushing Sentry, Boom Sentry, Cold Sentry, or Energy Sentry, in that order.
Each Sentry type's priority depends on the proportions of specific Bloon types that are on the map, and can increase or decrease significantly if a special condition is met.
Crushing Sentry gains priority from Purples and Ceramics, and gains extra priority if there are no Crushing Sentries and at least 15 Ceramics.
Boom Sentry gains priority from Leads, loses priority from Blacks and MOABs, and gains extra priority if there are fewer Boom Sentries than Cold Sentries and at least 20 Pinks.
Cold Sentry gains priority from Pinks, Purples, and Ceramics, loses priority from Whites and Leads, and gains extra priority if there are fewer Cold Sentries than Boom Sentries and at least 20 Pinks.
Effectively, if there are at least 20 Pinks and there aren't enough other Bloon types for other Sentry types to take priority, it attempts to balance the number of Boom Sentries and Cold Sentries evenly instead of going all-in on Cold Sentries.
Energy Sentry gains priority from all MOAB-Class Bloons (including DDTs, even if it can't detect them), loses priority from Purples, and loses extra priority if there are at least 10 MOABs and no Crushing Sentries or Cold Sentries.
Effectively, if there are many MOABs and not enough of the other Sentry types to handle their Ceramic children, it pre-emptively de-prioritizes Energy Sentries until at least one Crushing Sentry and one Cold Sentry are ready. This doesn't happen with any other MOAB-Class Bloons, even though DDTs also have Ceramic children.
All Sentry types lose priority based on how many of them already exist on the map, though the difference this makes is very minor unless there are many Sentry Expert Engineer Monkeys on the map.
Because of this, and that it prioritizes Sentries in a specific order if the highest score is tied, it naturally cycles through each Sentry type in order if there are no Bloons on-screen that can modify scores (though attack speed modifiers, or other ways of adding and removing Sentries, can break the order).
Deconstruction allows the Sentries to deal +1 damage to MOAB-Class Bloons and +1 damage to Fortified Bloons (+2 in total to Fortified MOAB-Class). For Energy Sentries, this stacks with their inherent extra damage to MOAB-Class Bloons.
The Crushing Sentry fires spiked balls every 1.2 seconds that can bounce off blockers, similarly to Spike-o-pult, except they can't rehit Bloons after bouncing. Each spiked ball can pop up to 22 Bloons per shot (33 with Oversize Nails). It deals 2 damage, +4 to Ceramic Bloons, and deals Shatter-type damage (can damage frozen Bloons but not Lead Bloons).
The Boom Sentry fires bombs every second that explode on contact. Its explosion has 18 radius, dealing 4 damage and can pop up to 30 Bloons (45 with Oversize Nails). Explosions pop frozen Bloons and Lead Bloons, but not Black Bloons.
The Cold Sentry fires ice bombs every 1.5 seconds that explode on contact, similar to Cryo Cannon. Its explosion has 18 radius, deals 1 damage, has 24 pierce (36 with Oversize Nails), freezes 2 layers for 1.5 seconds, and applies Permafrost (50% speed reduction for 2 layers). It does not damage or freeze White Bloons, Lead Bloons, frozen Bloons, or MOAB-Class Bloons, and will never target MOAB-Class Bloons.
The Energy Sentry fires lasers every 0.57 seconds. It deals 2 damage, +2 to MOAB-Class Bloons, and can pop 8 bloons (12 with Oversize Nails). It can damage frozen Bloons, but not Lead Bloons or Purple Bloons. Compared to other specialized sentries, its lasers have the fastest projectile speed (280 units/second), but still slower than the main nailgun.
Sentry Expert is a decent mid-game DPS with its variety of sentry types handling Bloons all around, but its sentry spawning algorithm can be unreliable and struggles on rounds where spawned Bloons change faster than Sentry Expert can redeploy sentries (e.g. rounds 50 and 53); stalling towers like MOAB Shove can counter this weakness.
4-2-0's damage bonuses are helpful and is preferred if building into Sentry Champion. 4-0-2 can be beneficial for Sentry Expert in the mid-game due to its significant pierce bonus, in addition to Pin helping stop Rainbow Bloons and below.
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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. — Update notes
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Sentry: Spawn rate increased ― 7s → 6s
Range of all Sentry types increased ― 45 → 50
4-1-0, 4-2-0
Larger Service Area now grants +4 range to Sentries
4-0-1, 4-0-2
Oversize Nails now allows Sentries to pop frozen Bloons
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. — Update notes
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. — Update notes
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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.
Crushing Sentry: Damage to Ceramics increased ― +1 → +4
Crushing Sentry: Damage type changed ― Sharp → Shatter
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. — Update notes
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Main attack: Attack cooldown reduced ― 0.42s → 0.35s (attack speed increased by 20%)
4-2-0
Sentry: Deconstruction's bonus damage to MOAB-Class Bloons and Fortified Bloons are no longer mutually exclusive
Improvements for Sentry Expert AI based on your feedback. — Update notes
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Sentry spawner: Priority given to Cold Sentries based on the number of Pinks now caps at 20 Pinks; reaching this threshold gives more priority to balancing the number of Cold Sentries and Boom Sentries
Sentry spawner: Priority given to Energy Sentries based on the number of blue MOABs now caps at 10 blue MOABs; reaching this threshold gives more priority to balancing the number of Crushing Sentries and Cold 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. — Update notes
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Overall cost reduced by $50 (due to cost changes for previous tiers)
Sentry deployment: Sentries now spawn within 20% of their range to the track
This was becoming annoying screen clutter with large numbers of sentries, and feels unnecessary when you can overclock the engineer itself to create more sentries instead. — Update notes
”
Overclock and Ultraboost abilities no longer target Engineer sentries
Really only one major change here; a rebalancing to how the 2xx Crosspath applies across various engineer upgrades, and some base stats being changed to accommodate fitting this. — Update notes
Sentries deploy with different damage types based on your need.
Crushing Sentry: Attack cooldown reduced ― 1.25s → 1s (attack speed increased by 25%)
Sentry: No longer gains Camo detection after loading a save (bug fix)
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 with 400 crosspath
Sentry Expert with 410 crosspath
Sentry Expert with 420 crosspath
Sentry Expert with 401 crosspath
Sentry Expert with 402 crosspath
Boom Sentry attacking
Energy Sentry attacking
Cold Sentry attacking
Crushing Sentry attacking
Portrait
Crushing Sentry portrait
Boom Sentry portrait
Cold Sentry portrait
Energy Sentry portrait
Upgrade icon
Insta Monkey icon
Crushing Sentry
Boom Sentry
Cold Sentry
Energy Sentry
