I will start. Since the meta is crossbows and knights, walls being able to block all arrow fire (friend and foe) that originated from soldiers that are not elevated and didn’t have a mechanic called arching shots that caused decreased accuracy (notably given to longbows) would get blocked by the wall, and the wall would take no damage.
For knights, if villagers could work the ground by say… scattering caltrops, that would make knights impractical. For reference, while a cavalry is on ground that has caltrops on it, their attacks are fumbled, and a fumbled attack does 0 damage. The result being that a person could lay caltrops in front of palisades (or in front of stone walls) and cavalry would be unable to break through.
I think this would shift the meta towards men at arms, and perhaps even siege towers. Speaking of siege towers, if villagers could construct them much like how mule carts are constructed, they might actually be built for once!