What is the difference between sentry and fortify with zone-of-control? Artillery, for instance-- They will fire when an enemy enters their ZOC, but what do they do if commanded to be on "sentry" duty? And... "sentry only enemy" is different than just plain "sentry"- does that mean that ALL units would be attacked? I've played this game a lot so far, and cannot find a real difference between fortify and sentry... Unless I'm doing something wrong?