It makes sense for a palisade wall to be >not< seethrough. Would you be able to look through a palisade wall in real life? Not really, its logs next to each other, bound together by rope, there aren’t really many holes you could look through. It would also make it easier to just stab or shoot through the palisade wall, if there were holes, or, in our case: Zombies could grab you through it. Slimes and some other creatures might even be able to simply pass through it, if there were holes.
It also makes, at least to me, little to no sense to use palisade walls for your base. You can look through, yes, but so can the enemy - zombies and other enemies would get aggroed very easily, and your base would be under siege in a matter of minutes (after all, it only needs a single zombie bashing at your wall to call more zombies over.).
I do agree with the Idea of making a plattform, ladder or something similar, basically a makeshift tower, that gives the player the ability to look OVER walls and even shoot or throw stuff over the walls. Don’t know how feasible it is though, I’m not a Coder.
As to firing ports…you mean embrasures or Arrow slits? if you make walls by yourself, definitely, this would be a feature that one should be able to do. I don’t know if its possible to make it passable for projectiles but not for creatures though. I don’t should be able to add arrow slits to premade walls though. Might be hard to do, and you could end up making a window instead if the wall just decides to start breaking a bit too much. Depends on the WAll I guess?