Sentry

Description
The  is a tower in Bloons Monkey City, Bloons Monkey City Mobile, Bloons TD 5 Mobile, Bloons TD 5 Deluxe, Bloons TD 5 Steam, and Bloons TD Battles Mobile, and Bloons TD 6.

Bloons Monkey City
The Sentry is a sub-tower spawned from a Monkey Engineer with at least the Sentry Gun upgrade. They will spawn every 10 seconds, and last 25 seconds before disappearing. With the Fast Engineering upgrade, the spawn time is reduced to 6 seconds Additionally, in BMC and its mobile counterpart, they suffer a bug where it is missing a line of code,, so Sentries can be deployed anywhere, including on water.

Upgrades
The Sentry has no visible upgrades. However, it internally has one upgrade, that being Sprockets. The Sprockets upgrade increases the firerate of the Sentry by 27%. All other upgrades have no function, though the Sentry's upgrades will match the upgrades of its parent Engineer.

Bloons TD 6
The Sentry is a tower spawned from an Engineer Monkey with the upgrades Sentry Gun, Faster Engineering, or Sprockets. Production rate increases with the second option, and +66% attack speed for the third. Most of the Engineer's upgrades will not pass on their upgrades to their respective sentries, but any external buffs onto the Engineer will buff the production speed of all sentries.

Upgrading further to Sentry Expert or Sentry Champion will spawn different types of sentries. Both of these upgrades will produce sentries with attacks that are far distinct from the standard green variants.

SentryTier4
Also called "Super Sentry", all sentries will become SentryTier4s when Engineer's Workshop is upgraded to Tier 4. These sentries spawn every 20 seconds and last 37.5 seconds. These sentries also shoot twice as fast.

Crushing Sentry
This is the first of the four Sentry variants that the Engineer can spawn after receiving the Sentry Expert upgrade. It's a red Sentry that shoots spiky balls which have high popping power and deal +1 damage to Ceramic Bloons. Does not damage Leads and DDTs without support. 

Boom Sentry
The second of the four Sentry variants spawned from Sentry Expert. This one is a black bomb-shooting Sentry. Cannot pop Blacks, Zebras, or DDTs without support. 

Cold Sentry
The third of the four Sentry variants spawned from Sentry Expert, colored light blue and able to shoot snowballs that freeze Bloons. Does not freeze White and Zebra Bloons just like Ice Monkey without M.I.B. It also cannot freeze Leads either without M.I.B. 

Energy Sentry
The final of the four Sentry variants that comes with the Sentry Expert upgrade. This one is a yellow Sentry that shoots laser blasts and has a high firerate but low pierce. As it is a laser-shooting sentry, it cannot pop Lead and Purple Bloons without support. 

Champion Sentry
This type of Sentry is spawned by Engineers with the Sentry Champion upgrade. It's a purple Sentry that is very powerful but also unstable; it shoots plasma at Bloons and explodes (causing up to 100 damage to up to 50 bloons) instead of just disappearing once its lifetime expires. Its weakness are only Purple Bloons and Camo Bloons. 

Trivia

 * Like all Path 1 upgrades, except Sentry Champion (cannot be picked up as Door Gunner), picking up any type of Engineer (such as one from a Sentry Expert) with a Special Poperations with the Door Gunner MK will cause the Engineer's spawn of Sentries to spawn around the Engineer's original range prior to picking up. This is likely a bug, but the Engineer will correctly place sentries around his new range radius once re-deployed back to a new location.
 * Since Version 16.0, a visual change was made to all Sentry Guns in BTD6 whereby the tripods rotate with the sentries' own bodies instead of only rotating their own bodies.
 * Boom Sentry has the largest barrel compared to its body out of all sentries in the entire franchise.
 * In the BTD5 generation, sentries consume around the same footprint as a Dart Monkey. The only exception is Bloons TD Battles Mobile, where sentries no longer consume footprint as of Version 6.9, following similar rules compared to Bloons TD 6.