@Aldarionn- I do believe that is what is eventually supposed to happen. In fact the beginning of Book 3 talks about just that happening basically.
What I think the OP was getting at was Drezen BEFORE the PCs & their Paladin army arrives.
Indeed that is what I was referring to, the status of Drezen when the PCs arrive there not after conquering it.
It is my thought that yes, it is in fact basically an empty shell. And thinking on it, the way Changing Man described it would make sense too after having read The Worldwound Gambit.
Also, while those craftsmen might be with the enemy armies, why would they deal with the PCs seeing as they are either part of enemy armies.
I haven't read the Worldwound Gambit so I don't really know what is described there - any hints would be appreciated!
So... why am I asking for ideas? The most interesting thing with mass combat up to this point for my group is handling the prisoners. Some players wanted to release them, some wanted to keep them along, most suggested that they were executed and that caused a very interesting debate (other than the party's paladin and the cleric of Iomeadae all the others play characters extremely hostile towards demon supporters - they actually expected that the army of paladin should execute all prisoners!).
Therefore, conquering Drezen and having to deal with several, dozens actually, prisoners, most of whom were non-combatants (not really civilians but rather not involved in the fighting) feels like it would be leading to very interesting situations as well as makes sense for the setting - tending the needs of standing armies I mean. Furthermore, several of those prisoners, may actually have been slaves, so were probably liberated rather than taken prisoner. What happens to all these people?
As to why would demons care about the welfare of human and tiefling supporters, I'd say for several reasons. First and foremost because they'd want them around! Fear can keep them leashed for so much. Armies would need food and drink, wages and something interesting to spend that money on - read here gambling, drinking, drugs, whoring, you name it. Otherwise...
Furthermore, don't forget that demons *need* mortals to "sin". They need mortals to behave in such a way in order to achieve their greater goal, which of course, as noted in the adventure itself, is not conquering land. Demons spend their time trying to seduce mortals to their cause, wouldn't they try to keep those already seduced satisfied? Up to a point of course...
So... that's the background story of my thoughts!
As always, any ideas and insight shall be greatly appreciated!