As walls have grown more advanced, people have discovered the need for bigger and stronger siege equipment. Thus enters the Trebuchet, with much higher strength, allowing it to decimate Ancient and Medieval Walls. This unit is a must-have for any civilization interested in mid-game conquest, as Catapults have quickly grown obsolete.
Keep in mind that, like all siege-class units, the Trebuchet cannot move and shoot on the same turn without the Expert Crew Promotion or boosts to Movement. For more details on how the move-and-shoot rule works, head here.
Artillery can be understood as a process of tossing bigger and bigger rocks at people. And for that, the trebuchet was the ultimate rock-tosser, at least until the widespread use of gunpowder. A catapult uses tension within a series of ropes and arms, but a trebuchet uses a lever and the force of gravity. The concept is pretty simple: a weight on one end and a sling on the other carrying its payload of stones (or even rotting carcasses to spread disease, or incendiaries to start fires). When the weight is allowed to drop, it swings the other end of the arm up and over, whipping the missile towards the target. A simple design, but one that required knowledge of levers and basic physics to perfect.
The trebuchet was used in siege warfare in Europe in the 1100s until the advent of gunpowder, and in Asia were used occasionally in addition to smaller, more mobile “crouching tiger” siege weapons. Trebuchets, being large and unwieldy, were usually built close to the site of siege, making armies lighter but making the process of besieging a castle a lot more work.