Okay here is the thing EAAgatha. Walls are no longer a safe thing unless there is a top over it. What I mean is player can now use a grapple/Fishing rod with grappling enhancements to have players get over your walls. No matter how tall no wall is safe. The best bet is to build walls then place a wooden trapdoors or slabs over any open surface. This way even if they do get onto your wall they cannot get into your claim. They will only be able to get on top of that wall cover of trapdoors or slabs.

Sincerely, VladDracul of Isengard

@EAAgatha, to ensure your walls cannot be easily scaled by a player with a grappling hook, I suggest making walls taller than 15 blocks and also having ledges. I have also found that placing a row of cobwebs atop your wall will stop grappling hooks (though this method can be tedious). It is important to remember that there are many other ways that players can get over your walls. For example, pirates may use a special cannon projectile, levitation potions, Shulkers, or even an Elytra to get over.
To build the safest wall possible, I suggest implementing my ideas stated above, as well as making sure you have claimed many blocks out from the wall. Sadly, walls will never be able to protect your base 100% of the time unless you build a giant roof over your entire base. Though, at that point, it would be a more of a box than a wall.