I'm not sure why people harper on about energy so much. It's far more to do with momentum transfer or even pressure exerted. You could transfer as much energy as a trebuchet does by lightly warming a large section of wall, but it would have very little destructive impact.Guys- do the math.
Trebuchets didn't kill people- the things the trebuchet HIT killed people. Its a physics problem. Ever seen one of those thingies with the hanging steel balls that transfer momentum back and forth? That trick only works because the steel balls are So Very Hard that the energy transfer is extremely efficient. Build one with rubber balls and its not nearly as effective.
Trebuchets were not aimed at people, but at walls and buildings. A Stone hitting another stone is a near perfect trasnfer of energy.
If a stone from a trebuchet hit the parapet of a castle, The other side of the stones making the parapet would shatter into a thousand sharp shards of stone.
They DID bring down walls, even very thick walls, because of HOW thick walls were built. Thick walls were made by building two THIN walls a dozen feet apart and filling the space between with rubble. If you shattered out a section of the outside stone wall, then the rubble interior would SPILL out like so much sand. This would leave a nice hole thru which your next shot would sail to break out an even larger section of the inner wall.
The resulting pile of spilled rubble would form a Ramp up to the breach. This is how jerusalem was taken, thru breaches in the several sets of walls created by roman ballista.
Cannon did the exact same work, just a lot faster with a much more portable device. Only then did castle walls stop being tall ( hard to climb ) and start being LOW and ridiculously thick earthen works faced with stone.