Well, siege is a weekly event, while this event you can build and use siege weapons. Only with siege weapons you can brake through palisade and stone walls.
There is a castle stone in each disputed land, it generates a weekly tribute in spices, and you can get it if you have a castle. Or if you will break trough others castle and capture it. And the most important thing is that you can get tribute only after a siege hour.
Mining time is a daily event. You have to find a mining hole in a disputed lands and when the mining time starts, you can get an iron ore for free. You don't need special items or skills, you just have to kill other people who wants to get free iron ore :)