The human siege weapon, it does exactly what you think it does. Use catapults when attacking other players castles to aid in killing defenders on the walls or just kill the walls itself.

Research upgrades for catapults unlock at academy level 3

Tier 1: Fitness training? (better make our catapults run that extra mile otherwise it will get fat.) – flat bonus health increase, leads to silver catapults.
Tier 2: Combat Training – Flat damage bonus, leads to wall destruction, giving silver and gold catapults even more bonus damage against walls.
Tier 3: Advanced Survival Training – Reduces damage catapults take by a %, leads to siege plating which reduces damage taken from ranged attacks by 10%
Tier 4: Basic Fitness Training – bonus +% health, leads to gold catapults.
Tier 5: Advanced Lethality – Increases the critical strike chance of catapults by a %, leads to advanced agility training which increases catapults attack speed by 10%
Tier 6: Advanced Fitness Training – Bonus +% health, leads to advanced destruction, which further increases the damage of catapults against walls.
Tier 7: Reduce production time of catapults by a %, leads to grassland training, which gives +20% health and damage when fighting in grasslands
Tier 8: Training Discipline – reduces cost of producing catapults by a %, leads to research that reduces the cost of blood diamonds to produce catapults by 20%
Tier 9: Elite Combat Training – increase damage by a +%, leads to “Heavy Boulders” research which increases the attack damage of catapults by another 20%.
Tier 10: Elite Lethality – increases critical strike chance by +%, leads to Elite Agility Training which provides catapults with +20% attack speed.
Tier 11: Elite Survival Training – further reduces the damage taken by a %, leads to Reinforced siege plating which reduces ranged damage taken by another 20%
Tier 12: Elite Fitness Training – Even more +% health, leads into Honor Bombardment, which increases the honor gained when attacking cities…..this research is useless and you can skip it.