A siege is when you attack and weekend a castle or town until you can take it over the reason for doing this is to get resources and food so they don't have to go out and grinding for those resources and wait till the food grows.
The ballista was a ranged weapon made out of wood and use to weaken walls of castles they would use it to siege.ballista was an attacker and a defensive weapon also they would use it by pulling tension on the dart and then letting it go They also had attachments for the ballista like the ballista wagon. which made mobility with the ballista very easy. The ballista had one weakness which was water if it was even a slight rain or water of any sort the ballista gets affected negatively. Ballista's where powerfull enough to shoot 10 pound bolts
Sieges happnened because the attackes wanted free resources and