Thanks for all your comments. Seems like this is a genuine known problem in MTW. All of you seem to agree it is virtually, if not totally impossible to capture a leader inside a castle's defences when conducting the siege yourself, yet the autocalc, in a very high proportion of circumstances, does capture the leader in a castle siege.

I know from practical experience and it has also been discussed recently on this forum, autocalcing a castle siege often gives better results than you can usually achieve commanding the attack yourself, especially if you do not have siege equipment. The total reverse to autocalcing battles in the field where you can usually do far, far better yourself than the autocalc.

So I guess this is another aspect of the autocalc where it does a less than stellar job compared to what happens when you fight the siege attack yourself.

Tomcat