Basically these are units with the highest attack among all troop types, such as the Mighty Trebuchet(X), with 164 attack. All of them have very slow moving speed. They require less food to train than the other troops but require more wood to train. They are trained in the Siege Engine Factory, unlocked at lvl 5 castle.
These are best used with meat-shields(Infantry or/and Cavalry) as they have low attacking speed and very long ranged attacks. They should not be deployed alone as their slow firing speed gives an opportunity for enemy troops to deal damage to the chariots, which mostly have low health(Compared to the other troop types of the same level)
These troops can carry the largest loads, such as the Assault Cart (VI). Remember this when farming.