So I chose the non-imperial own Kingdom way for my playthrough. I defeated the western kingdom before the quest reached the threshold to trigger the all out Empire faction war and I think this is where the problem lies. I defeated the southern and northern kingdom, even though to defeat the...
The enemy was outnumbered by a large margin and a flock of 200 men tried to kill one single soldier which was pushed into the wall and thus couldn't end the siege. Had to retreat and attack again.