Regarding the catapult, the stone bounces rather than rolls. I have experienced a few goblin sieges where I had two legendary siege operators(and one accomplished) firing two catapults continuously at a mass of goblins(for some reason they never tried to enter except when I tried to lure them in), so I got a pretty good chance to study the mechanics behind the catapult. If you watch the stone travel you will see when it touches the ground. The stone slows considerably and leaves a patch of mud behind. The stone then bounces two or three more times before stopping and becoming a
>. Studying the damage inflicted upon the monsters in the stone's path suggests that the projectile travels in a genuine arc. Monsters situated on "ground zero"(the exact spot where the stone lands first) will sustain massive damage, almost certainly lethal. Even trolls will usually die outright and be propelled away, but I haven't hit anything sturdier than that. Subsequent landings will also deal great damage, although perhaps less than the first bounce. The stone is, however, far less lethal between landings, although still dangerous. Here the stone will usually only break limbs, sometimes crippling them and sometimes only just bruising its targets and may even bounce over the enemy altogether.
Another thing should be observed when using catapults is that the operator adjusts the siege engine's range to hit the nearest hostile creature(that is, not wilderness). However, the operator does not factor the firing arc of the engine. This means that the catapult shots may fall far short of a mass of goblins standing across the river right in front of your gates if there is another group of goblins standing by the cliffside at the upper edge of the map.