Make it more hp or nerf arrow damage on it. UIF archers can destroy it in no time and shield wall is kindly useless on siege because of high wall that allow archers to concentrate fire on kindly big catapult.
Ah and when I use my wooden material to reload catapult(uber poweranger ninjastorm transformation into fuking stone).... it is silly,
Lets make different catapult amo kinds,
profit is really big because you can adjust prizes on it.
I suggest 3 kinds of amunition:
Stones (against walls)
smaller bags of stones that can attack infantry(not always kill but lot of damage on some useful area)
and burning rooks, most expensive, something between stones and that anty inf shit(can set some area in fire for some time)
like always it is only idea,
have nice day