The war field need to be clutter whit units, it is a basic
It is the people who make it alive! They have will, they have fealings. They are animated.
If you hit you best friend, it will mostly end you friendship and start a war( figth) betwenn the two.
Same goes on the batle field, if your units are too much close on each other and push between them, they migth start pushing each other to make place and end up killing each other, even if they are allies.
Objects dont move, they are death. They have no will and belong to the one using it.
A wall belong to no one, untill you set a person on it, to see far away.
Also, rock are inanimated. A trowed rock hurt any one and dont move by it self.
I trowed a rock on my best friend, at the place of my enemy. I hurt my friend and my friend became my enemy. My old best friend, now my enemy, became neutral to my enemy. I lost good time vs 2 oponent, insted of being a easy victory.
Machine dont move, they are death. unless a outside force activate it. They have no will and belong to the one using it.
A active trebuchet, whit people using it, can fire at long range, depending on elevation ( a trebuchet on hill or wall will fire more far off then a trebuchet on plane land).
But a active trebuchet, whit no one activating it, will just be a desolated useless building, taking presious place. ( whit a villager, maybe recycle the thing for wood? Or even learn the villager to use it?)