So, there is a description by a king to his son " to resist the trebuchets, the cat, and the engine called the ram, it is well to strengthen the entire stone wall on the inside with large oaken timbers, though if earth and clay are plentiful, these materials had better be used. Those who have to defend castles are also in the habit of making curtains of large oak boughs, three or even five deep, to cover the entire wall; and the curtain should be thoroughly plastered with good sticky clay."
So, with this, we have the basis for actual medieval defenses against trebuchets. In the game, if you surround a castle with stone walls, it will be resistant to melee, (not rams though sadly) but it won’t be resistant at all to trebuchet shots. Same thing for archer arrows. They can fire over walls with impunity.
But in this medieval text, we see that castles hung boughs made of wood on the outside of the castles to blunt damage from siege weapons. What if palisade walls gave extra armor to structures within 1 tile of the palisade wall?
The palisade wall would have a usecase in the lategame for castle defence if this became a thing, and attacking trebuchets would need to be guarded for longer while they attacked to successfully knock out the castle. Keep in mind, a treb is 200 wood and 200 gold, and a castle is 650 stone. Wood is plentiful, and gold can be made from trade or holding relics. Once that stone is lost, it is gone for good. Giving players a way to defend that lost stone is kinda important.