What is a siege?
When you siege a castle, it means you are attacking and raiding a castle for loot and to conquer the castle. A castle siege was done because people wanted to capture and loot other castles.
The original ballista, built in 4th century b.c. was built to toss big stones and bolts. By the medieval ages it was a very good offensive weapon that could break walls or launch things such as dead people into the castle.
Eventually giant arrows became main ammunition for the ballista and they had so much power that the arrows broke walls.
The word Ballista literally comes from the Greek word ballstes, which means to throw in english.
For the times it was very impressive because a ballista could launch up to a 10 pound spear into the enemy.
A ballista was so big that it had to be built on the battlefield. So engineers would go on all of the seiges to build these ballistas