How do I get it so the knights that are loyal to me will attack castles with me? I've looked all through the tasks and no where is there an option to attack a castle.  I've also told them to follow me but when I attack a castle they do not attack with me.

If they're in good standing to you they will join the fight. Do some quests for that lord so he likes you or have a lady increase your relationship with him (you'll need to pay up for that, and you'll need to be in good standing with that lady first by having defended her honor in a quest). I dunno why you joined this site to ask that, not many people play this game here, try the official community.