A siege is a form of plan to contain a large area in the middle ages. The siege was an act or process of trapping and attacking a secure place so they couldn't get essential supplies they needed to survive. The purpose of lessening the resistance of the defenders was to make capture possible.
A ballista is a tool that was created in the medieval times. The ballista was designed to be a weapon as an attack mechanism against that was used against enemies during wars and battles. The ballista looked liked a huge crossbow.
A ballista was similar to a crossbow because of it's look and fighting style, but the ballista has more power to damage it's enemies by firing deadly, sharp and big objects that can burst through iron armor or chain mail. The huge wooden weapon was used in wars and battles to shoot enemies from far away. The crossbow like weapon could fire a large wooden arrow, dart or iron clad and could shoot about 500 miles. The ballista was made out of fused wood and animal sinew, the whole thing was wrapped in animal hide.