Graeme Dice said:
The evidence is in the manual in the section on sieges.
If you've followed the thread about the mechanisms of dominion spread, you know the manual may not be always right. Castle defense is not supposed to use any randomness, so checking that it works as described should be possible.
Played with it last night. The manual appears correct.
It's hard to see since the current defense strength is only shown while you're actually besieged, but I knocked the walls down most of the way, moved away, then back. Defense strength was still low.
Probably should check what happens when you actually take a castle.