This is kind of the skeleton. It's a bit like a spider: the player fights from within the body, the legs extend to the wall and down into solid ground uninterrupted. The weak part("weak": they're made of steel both for SI and durability) of the legs are elevated, wedge-shaped and protected by those wedges jutting out. The zombies fall straight into the pit. The base of the legs are protected by several layers. This one can continue standing with 5/8 of the legs destroyed.
https://imgur.com/a/bSBKVCb
https://imgur.com/a/ax3weqB
Inside the "body": There are two platforms for fighting. The lower one has openings in the middle and at the corner for flinging bombs and molotovs down. There is a box, shielded from below, for extra supplies(typically mostly water for molotov mishaps.) You can only fall down if crouched(feature). The upper platform is level with the ground approaching the pit, good for another player to take out priority targets(cops) or kill and slow down groups.
https://imgur.com/a/wbvOMsc
They fall into a pit of reinforced concrete wired up with electric fences, dart traps, and turrets that I use in that order if things get a little too hairy. I rarely have to activate the darts or turrets, but use the fences quite a bit as they are strung next to the walls and keep them from damaging too much. I still use a compound bow for a good amount of kills.
Finally, I will share two pictures of a base I've been building anticipating the demolisher. The main differences are: only one floor within the body(so I can keep an eye on the approach while playing alone) and it's much easier to enter and exit in case I need to go outside to kite a demolisher away. Oh, and I would probably make a lot of asphalt for the approach ground.
I am reluctant to share them because, I warn you, this one is cheesy. It takes advantage of gaps that I know bug a lot of people. It honestly bugs me in this particular case, too-this is just the first design. The base above began like this and this one is being redesigned with the same wedge legs. I also prefer for it to be realistic, even when it's not required. So, please don't hate on the cheesiness. I haven't even horde tested this one.
https://imgur.com/a/ZgaAH5f