How many layers of concrete makes a 'castle' to you? I'm on two already, but it's barely enough.
Multiple layers of concrete walls with metal bars in the centre two rows, to hold the Z's off for long enough for you to kill them. Even then, you're going to lose blocks all around your outside.
Feels kind of crappy, since there aren't a lot of viable base designs, but I've tried all sorts of stuff like ladder/hatch-only access to my base, or living on the top floor of a tower, living in an underground bunker, windows on all sides, windows on only one side, ect.
Now that they zombies can path-find better, they will undermine towers and dig down into bunkers, and attacking from alternating sides on horde-night is a problem if you place all of your defences on one side.
Z's block damage is just too high for a regular building to stand up to them for the full duration of horde-night, unless you have enough players with guns (or turrets) to keep the zombies from piling up against the walls.
Knocking block damage down to about 30-50% in the settings stops them from just drilling though your cobblestone/concrete walls in seconds, but it lowers the tension in POI's, because they can't get out of locked rooms, and you can just close doors behind yourself as you move about to buy yourself time to heal or reload.
... which makes sense, I guess.
Maybe if the block-damage stayed high, but there was some sort of cooldown, preventing multiple zombies from being able to strike the same block simultaneously?
As is, a z-pile grinding against your walls can hit the same block ten or twenty times in about two seconds, which drains the block-hp from max to broken in seconds. Ensuring that they
can't pile up like that is absolutely vital.
Putting spikes against your walls has them not be able to hit the block behind the spikes, so they break holes through your walls at head-level, which creates a nice shooting porthole, but it's probably not an
intentional solution.
As-is, we generally ring the base with multiple layers of electric fence, to try to keep them away from the walls as we kill them, but that means spending tons and tons of ammo.
Personally, I kind of want to try a five-layer wall on three sizes, to see if that deters the zombies from even attempting to get in from those directions. That'd be a huge pain to gather resources for and upgrade though.
Could debugmode it, i guess.