Sentry Expert

“	Sentries deploy with different damage types based on your need.	”
	^{~ BTD6 description}

Sentry Expert is the 4th upgrade of Path 1 for the Engineer Monkey. It allows the Engineer to create specific sentries that specialize in popping specific bloon types. The Engineer may produce specific types depending on what bloons spawn within or near his radius.

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.

Sentry Expert costs $2125 on Easy, $2500 on Medium, $2700 on Hard, and $3000 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)

Crushing Sentry

The crushing sentry appears red and shoots spiked balls reminiscent of the Dart Monkey's Juggernaut upgrade but with the popping power of a Spike-O-Pult. It has the highest popping power of any sentry. These balls deal 2 damage to Ceramic Bloons and are able to bounce off obstacles.

Pierce: 22
Attack Speed: 1.00/s
Damage: 1 (+1 to Ceramics)

Does not spawn if Lead Bloons are within range of the Engineer, but it will spawn if there are Ceramic Bloons nearby.

Energy Sentry

The energy sentry appears yellow and shoots laser blasts reminiscent of the Super Monkey's Laser Vision upgrade. It has the highest attack speed, however the lowest pierce of any sentry. Laser blasts can pop frozen bloons.

Pierce: 4
Attack Speed: 1.75/s
Damage: 2

Does not spawn if Lead Bloons, Ceramic Bloons or Purple Bloons are within range of the Engineer.

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. Cannot affect Whites, Zebras, or Leads.

Pierce: 15
Attack Speed: 0.67/s
Damage: 1

Does not spawn if White Bloons or Zebra Bloons are within range of the Engineer.

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.

Pierce: 30
Attack Speed: 0.77/s
Damage: 2

Does not spawn if Ceramic Bloons, Black Bloons or Zebra Bloons are within range of the Engineer, but will spawn if there are Lead Bloons nearby.

Tips

Just like Laser Blasts, the Energy Sentry can't pop Lead or Purple Bloons.
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.
The Crushing Sentry deals double damage to Ceramic Bloons but can't pop Lead Bloons.
Even though it cannot affect them, cold sentries will still spawn if there are Lead Bloons.
- There is a bug/special case where only cold sentries will spawn when there are DDTs present.

Strategy

Sentry Expert is a situational upgrade for upgrading the sentries. On certain aspects, it can be less powerful, due to the fact that most of the Engineer's special sentries attack considerably slower, with exception of Energy Sentry. On other aspects, it can be pretty helpful, as most of the special sentries utilize a combination of utility and extra overall damage in some way shape or form.

Certain sentry types can be very useful in different situations:

Energy Sentries are powerful against damaging MOAB-class bloons and popping up bloons for a rather low price.
Cold Sentries are helpful at stalling Ceramics, especially Super Ceramics, a feature that Pin is unable to do.
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 can deal a decent amount of area damage along the way.

However, there is very limited control over the Sentry Expert's production of certain sentry types. As such, 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.