A siege means where a army surrounds a castle and tries to get it by attacking it. Doing a siege was either to conquer more land or just for fun.
A catapult was first made in the medieval times as a long range weapon to be used in battle or in sieges.
Both catapults and ballistas work by storing tension either in twisted ropes or in a flexed piece of wood.
A catapult is a ballistic device used to launch a projectile a great distance without the aid of explosive devices.
Extra Credit: A medieval castle had a lot of unique features. For example, they had moats which is water surrounded the castle making it harder for sieges. They also had dropping castle doors which most modern day castles don't have.