If "sentries" refers to guards:
* Defenders: This implies a more proactive role, focusing on actively protecting something rather than just standing watch.
* Attackers: This is the complete opposite, referring to those who are trying to take something, rather than protect it.
* Invaders: This implies a hostile force entering a territory, the antithesis of a sentry who is defending it.
If "sentries" refers to something that watches or observes:
* Observers: This is a neutral term that implies a passive role of simply watching.
* Participants: This implies active involvement, the opposite of a sentry who is simply observing.
If "sentries" refers to a specific type of guard:
* Prisoners: If "sentries" refers to guards in a prison, the antonym could be prisoners, as they are the ones being guarded.
* Civilians: If "sentries" refers to guards on a battlefield, the antonym could be civilians, who are not involved in the fighting.
Ultimately, the best antonym for "sentries" will depend on the specific context in which it is used.