Like it says in the title. More than once I've settled troops in for a quick siege without having assigned generals beforehand, then looked up to see a (usually allied or mercenary) doomstack coming in to ruin my fun. My problem is, I can't seem to figure out how to get into the flank/general interface of an army that's in the process of a siege. So I end up fighting a defensive battle at a severe disadvantage. Am I missing something, or is it working as designed?