Just bought the game and have been studying the manual and am confused on one point. I understand that a siege weapon ads 4 to your attack strength when marching into an area with a castle. However when it is a neutral castle without a garrison why is it that the castle doesn't provide any points to defense. It would then seem a disadvantage to have a castle when siege units come a long and no advantage to defense when being attacked by normal units?? Why would siege weapons be so powerful when attacking castle/fort yet the fort offers no amount of defense points. Or have missed something in the manual?? Thanks for any help.

No you haven't missed anything. Fortresses and Strongholds provide no defensive bonus.

It's a strategic reason, not a thematic one -- Castles are already difficult enough to capture, and giving a defensive bonus encourages turtling, instead of expansion.

Ok well that makes sense then. Thanks for the quick reply!