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In dom2 if the castle ever gets un-besieged it will revert to full strength immediately. Not sure if it is different in dom3 but I will believe it is the same until there is evidence to the contrary. The formula Fate gives for siege values is/was correct - each unit has a siege value equal to (strength^2)/100 + siege/defense bonus + flying bonus - mindless penalty and then the individual siege values of all units in the force are added. The defender's force is compared to the attacker's force and the castle walls increase or decrease depending on whose score is higher. So it is possible to have a small force of elite units that can easily defeat the enemy in the field but cannot break into the castle. Of course, the defenders inside the castle may starve eventually.

I do not remember if mindless penalty and flying bonus work on both sides of the equation or not. I think flying works on both attack and defense (+1 to the individual unit's total score if he flies) and mindless counts only on defense (-1 to the individual unit's score if it is mindless).