This engine was of much more recent invention than the catapult or the ballista of 
the Greeks and Romans. It is said to have been introduced into siege operations by 
the French in the twelfth century. On the other hand, the catapult and the ballista 
were  in  use  several  centuries  before  the  Christian  Era.  Egidio  Colonna  gives  a fairly accurate description of the trebuchet, and writes of it, about 1280, as though 
it were the most effective siege weapon of his time. 

The projectile force of this weapon was obtained from the gravitation of a 
heavy weight, and not from twisted cordage as in the catapult and ballista.
From  about  the  middle  of  the  twelfth  century,  the  trebuchet  in  great 
measure  superseded  the  catapult.  This  preference  for  the  trebuchet  was  probably due to the fact that it was able to cast stones of about 300 lbs. in weight, or five or six times as heavy as those which the largest catapults could project.¹

The stones thrown by the siege catapults of the time of Josephus would no 
doubt  destroy  towers  and  battlements,  as  the  result  of  the  constant  and 
concentrated bombardment of many engines. One huge stone of from 200 to 300
lbs.,  as  slung  from  a  trebuchet,  would,  however,  shake  the  strongest  defensive 
masonry.

The trebuchet was essentially an engine for destroying the upper part of the 
walls of a  fortress, so that it  might be entered by  means of scaling ladders  or in 
other  ways.  The  catapult,  by  reason  of  its  longer  range,  was  of  more  service  in causing havoc to the people and dwelling inside the defenses of a town.

From experiments with models of good size and from other sources, I find 
that  the  largest  trebuchets--those  with  arms  of  about  50 ft.  in  length  and 
counterpoises  about  20,000 lbs.--were  capable  of  slinging  a  stone  from 200 to 
300 lbs. in weight to a distance of 300 yards, a range of 350 yards being, in my 
opinion, more than these engines were able to attain.²

¹ The catapult had, besides, become an inferior engine to what it was some centuries before the trebuchet was introduced, the art of its construction having been neglected.

²  Egidio Colonna tells us that the trebuchet was sometimes made without a counterpoise, and that in such a case the arm of the engine was worked by a number of men pulling together instead of by a heavy weight. I cannot believe this, as however many men pulled at the arm of a trebuchet they could not apply 
nearly the force that would be conveyed by the gravitation of a heavy weight.