I love to use mortars when sieging castles. Once you have put a hole in the wall the AI usually moves most of its garrison into a big huddle inside the second wall. If you don’t send anyone in through the breach this group hug just stays there.

There is usually a point down the side of the castle (where they are just in range), that you can position your mortars to fire over the walls.

The ideal target.

Two or three mortars can take out halve a stack of defenders in this position.