Take a small building such as an Oxen Tether. Place one or two of them directly outside the gatehouse(s) of your enemy's castle, but make sure their keep is enclosed first. Immediately put the Oxen Tethers to sleep. The ground troops will walk to the gatehouse to destroy them, but cannot get through. They will just walk back to their spot. Your enemy's buildings outside of the castle will spontaneously crumble because the workers cannot get back inside. Also, the gatehouse(s) that you blocked may spontaneously crumble after awhile. Soon there will be no food and the peasants will leave the castle. The small army will be trapped inside their own walls awaiting your arrival. When your army is built up, you can move over and wipe the whole castle off the map. Note: Make sure that there are no enemies outside of the castle, or else they will probably destroy your Oxen Tethers.