Rock, scissors, and paper are applied in the battle of units. Foot unit is powerful than a bow unit, bow unit is powerful than a horse unit( if the horse unit is attacked indirectly, you will do more damage) and horse is powerful than a foot unit.
To get this siege weapon, you must have Jiang Zhou under your control. Be a prefect, if necessary. This place has the perfect facilities to build juggernauts, Machine Yard and a Foundry. Juggernauts are pretty powerful because it can damage enemy unts in the next two spaces. Remember, allies in the two spaces get hit too.
First be a viceroy. Then corrupt then the cities that are under your control. Leave no gold. Then resign or revolt. You can request the ruler (ex. Cao Cao) to widen your control (ex. 2 cities to five etc.). You will resign eventually so get all the gold to buy items in bazaars.
When you are travelling, sometimes, you encounter a bandit or an item. When you encounter a bandit, do what fits for him. After a few turns, you noticed that the area where the bandit that you defeated havenow an item or perhaps gold.