Below are the changes to castles and strongholds and their reasonings:
Castles and strongholds have an inherent defense bonus to initial combat strength of 1.
(this needs no explanation, if you believe that hiding behind 6 feet of stone and shooting arrows through "murder holes" from 40 feet in the air at your enemies confers no better defense bonus than hiding behind a bush in an open field then seek help)
Siege engines may be taken as casualties in combat, but may never retreat.
(defending or attacking forces may hide behind their siege towers or ballistae as cover, but that cover may be destroyed in the process)
Siege engines have a defense of 1 if it is in a castle or stronghold, but may not retreat or be taken as casualties from special cards or sword icons.
(you have comitted your new artillery to the outer walls of the fortress to improve defenses but now are part of the fortress now)