Sentry Guns can be sold for $0, and also have a target priority which can be changed. Often Sentry Guns would only be sold to give space for another tower to be placed on.
They are effective on daily challenges and special missions where the amount of towers is limited, as they do not affect the total tower count.
Sentry Guns are created every 10 seconds and have a lifespan of 25 seconds.
In the case of BTD5 Deluxe, Bloons Monkey City Flash and BTD6, Sentries' target priority is determined by the Engineer who has spawned them. In BTD5/Battles/Monkey City Mobile, however, Sentries default to First no matter what target priority the Engineer has.
When you upgrade the Engineer to Sprockets, any new sentries spawned will have the name "SentrySprockets". These sentries are not affected by Monkey Knowledge in BMC Flash.
In the mobile versions, the sentries are instead internally upgraded 0-3, and are affected by Knowledge in BMC Mobile.
The Sentries actually might be based off of the Sentry Turret from Valve's Team Fortress 2.
The BMC Mobile, Sentry is missing the "OnlyTargetPlacementLocations": true line of code it has in BTD5 and Battles, causing the sentries to spawn anywhere, including on the track, water and unplaceable spots.
When a sentry runs out of time, it explodes. The explosion is visually the same as the Splodey Darts one from BTD5.
If the player sells the engineer, his sentries would be gone instantly after selling the engineer (Mobile only).