As to be expected, a cannon (the dragon in this case) often just destroys whole stacks of enemies with ease, often enabling me to kill 4-5 soldiers per round. To help prevent or at least make it more difficult to practically take a castle by yourself, make dragons/mortars/siege handgonnes, strength activated. If I'm lugging around a mini cannon, it should require a great deal of strength. Perhaps 12 for a dragon, 15 for a mortar, 18 for a siege handgonne.