Quote Originally Posted by Didz
The trigger clearly doesn't exist otherwise the AI would not be allowed to just sit there and do nothing.

As you say the only way to end the battle is to push EXIT or sally neither of which is an acceptable solution. What is needed is another 'Do Something' trigger like the one that forces the AI to make futile attacks on missile troops, but which in this instance forces it either to make a futile attack on the city walls or withdraw.
Im thinking this out here, and I am not suggesting its perfect, but if an army seiges a castle, looses a great deal of men isnt then it withdraws. The AI withdraws and waits for you to remove it via sally.

Now you claim sally isnt acceptable, i disagree as its completely realistic in a military sense, if your under seige eventually your capitulate or you sally.

Now the do something trigger, yes i think that should be in there. In MTW if I recall you could "mop up" the battle and the AI would auto calc it at that point based on the avialable forces left. I think thats reasonable.