So I noticed thieves were bypassing my army of guards then getting butchered as they made their way out. I wondered how they got in, if everyone else died. Then I saw it, my worker thralls opens the door, thief gets in, door closes, guards forget the thief exists. Either the workers need to stop using doors during a siege, or the combat thralls need to learn how to use their own doors to chase an enemy. Probably best if doors are left closed during siege events, and workers take cover instead of acting like nothing is going on.