Need help with Bridges!

My new map: Seige of Isareal, is comin along nicley, but i need a way to eliminate my bridge problem.
In my map i have a bridge that can be use as acrossing between sections of the wall.. I want to make it so that a orc catapult (and catupult only) can destroy this, openin a path for the orcs to slaughter the defenders. I just cant get this catapult to kill the thing! help!

make a new destructible. base it off of a gate model. make walkable, give bridge pathing and model. it should now be "killable"

And make a trigger to cancel any unit's order to attack it except the catapult, with a message saying "Only the catapult has sufficient power to damage this."