A siege is when enemies go to a castle with tents with a lot of supplies and camp outside of the castle. They wait for the people in the castle to run out of food and then are forced out of the castle because of hunger. The enemies will have to wait many weeks some time a year and then they attack the people in the castle when they are weak.
The battering ram was used against many castles so that enemies could enter and take over. It was a long heavy pole and would take many men to use it together for better strength. Today Swat teams use these to break down doors of bad guys.