1000 castle supply really is useful. Question about that...when you're seiging, is there any way at all to see what the supply factor for the troops on the inside is?
The Jervellan Wall has great siege supply, but not much defence, so unless you have a very tough army inside the walls will fall rather quickly. The idea (I'm guessing) is that it takes a lot of manpower to hold, and therefore has a correspondingly high siege supply.