Just played a learning game, can't see any mention in the rules of Castles & Strongholds doing anything but being mustering points. Is this correct?
It seems that maybe they should add to combat strength for defending armies, especially if the attacking enemy gets a benefit from siege towers. Otherwise holding an area with a stronghold must be a lot harder than holding an non castle/stronghold region with the same strength army. Haven't played properly yet so purely speculating...
The only castles/strongholds that get defence benefits are those with garrison markers still on them (i.e., everyone's starting stronghold and any neutral forces if you are playing with fewer than 6 players).
So to answer your question: No, normally castles/strongholds do not get any defence benefits.
I think it was intentional in the design of the game to make castles/stronghold easier to capture with siege engines. Siege engines do have their risks as they can be easily caught out of position and killed so just keep that in mind.
Good luck in your future games!