"Sticks and stones will break your bones" is a great motto when fighting skeletones. This takes care of the "stones" part. Plus, the more you invest, the more you get.
— Game Description
The Wall Catapults' weapons are rocks, heavy rocks, the same rocks that are thrown in Catapult Volley.
The Wall Catapults are in the center of the Castle Walls, helping to defeat incoming enemies with the large boulders. This units can't be damaged or repaired.
When upgraded, the Wall Catapults throw boulders in fire, making them more efective. The Max Wall Catapults you can have are 3.
The Wall Catapults are some of the few of the units that can't be summoned.
The Wall Catapults have no Defense, Life, or cost (In Iron Bars).
The Wall Catapults are a Castle unit, which means they have to be bought in order to be used.
You don´t need the Catapult Volley skill in order to have the Wall Catapults, or vice versa.