One of the new 'red hand' mods in Dungeon mag has the town under siege, but I think it was just for a few encounters. This could be adapted to fit a castle. I bet the 3.5 edition of the Red Hand stuff has this as well. I also seem to remember a 10+ year old dungeon mod involving a underground siege.
A good idea could involve a castle siege and the party needs to get in or out, around the siege to deliver a message, rescue someone, kill someone to stop the siege. Something like in the Michael Criten book/movie Timeline.