Select an army but instead of moving it to a province move it over the castle. The colour should fade a bit (like a emissary or ninja when you choose a mission) and then let go. You'll get a battle confirmation screen like with other battles and at the end of the turn you'll fight it. You can only use one army afaik so put all the troops you want to use into one army before you select it. Depending on the troops castle defence can be costly. If it's after a batttle there is usually a good mix, if the enemy just retreated to the castle then you might find the defence is all archers or warior monks and you can select the relevant troops. There's nothing more fun that lining up your archers carefully outside a castle and shooting down an army withot a single casualty.
the troops in the castle rarely make a break for it, although sometimes the enemy will station troops outsde the castle.
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