I don't know for certain.
I think the most likely things is that the army will stay inside the walls, leaving the PD alone. In the movement phase, it will either move away or not, depending among other things on the phases of the moon.
There are some really strange things that can happen with the magic, movement and event phases. One of the more curious things I've seen happen is that I get a barbarian horde event in a province that has Niefeflheim's turmoil 3, sloth 3, death 3, misoftune 3 dominion and on the same turn Niefelheim puts my castle there under siege. Nothing in particular happens. The next turn I kick his pitiful forces out of the province. And right after the siege breaking battle, the barbarian horde from the turn before attacks me (and dies, of course).
The question for all and sundry: WTF, and who is responsible?!
Edi