A siege is when an army of people surround a castle and take down the wall. The army can dig under the walls and cause them to collapse. The army could use a catapult or a ballista.
These two videos will tell you how the catapult and ballista work.
As you could tell by the two videos the catapult and ballista are a lot more difficult to make them work than you thought. Catapults and ballista were really important back than. They were common weapons used during sieges in the middle ages.