Depending on how the fortress takes care of human waste, you might have another entrance into the fortress.
That said your idea seems like it could work, but it might not go as planned. I If there is information that Fort B is going to be attacked, Fort A is going to be especially wary of the possibility of it being attacked.
[[My personal thoughts]]
If Fort A sends troops to Fort B, then Fort A would probably have a heavier patrol during times when there is lower visibility. *
However I believe if Fort A was really cautious and/or smart, they would keep their troops that way if Fort B does get attacked then they could send troops to attack the besiegers from behind. This means that they will still have their full garrison, and have heavier patrols just in case the information is wrong. * [[/My personal thoughts]]
All in all you are writer, so you know your story better than any of us. If you think that something could work, then write it!
edited 8th Feb '11 4:37:38 PM by EldritchBlueRose
Has ADD, plays World of Tanks, thinks up crazy ideas like children making spaceships for Hitler. Occasionally writes them down.