Our group has built a similar thing-- we call them "murder corridors."
We have figured out some other interesting arrangements that allow us to have a ground level base with walls, and survive horde night by fighting zombies with little to no base damage.
The key thing is simply cutting off viable zombie pathing.
So, our murder corridor is a tower with two windows facing in two cardinal directions. Similar ramp and runway design toward the windows. Barbed wire laid down on the runway to slow zombies down.
The only way into the tower is from a bridge connecting the roof of our main keep to the roof of the tower. And the only way to get onto the roof of the keep is via a ladder that starts two meters from the floor. We'll eventually replace this with an exterior main staircase to a door on the second floor that will have a draw bridge to break it during horde night.
By cutting off the roof access with the two meter up ladder, so long as you are on the roof, preferably on the tower or in it, zombies that are hunting you will path through the murder corridors.
Our murder corridors travel quite a distance but then veer from the windows and wrap around to side access doors in the middle of the tower. And these veering paths are blocked off on the runway with 'obstacle' doors. This causes the zombies to get stalled at the runway door and start punching that, which gives us time to consider whether we are going to get overrun. If they break through the runway doors, we escape to the roof, or simply open our side access doors and jump down to the ground.
The side access doors on the tower can actually be toggled to cause zombies to reconsider their path. If you toggle the state of the access doors, any zombie that falls off the runway will then choose the other corridor, which allows us to juggle and manually distribute them between our fire teams.
We used to have an iteration that allowed someone to repair the obstacle doors. Currently, if we have a catastrophic failure that causes the zombies to focus on our foundations, then we bail on the tower and try to coral zombies away from the castle while others try and patch up the damage.
We're about to build a second murder corridor tower system on the opposite side of our keep that will basically allow us to have primary and auxiliary fighting positions. If one tower falls, we can rush to the other before the zombies start attacking our foundations.
It should be noted that we allow zombies to access our base. The murder corridors span from our outer castle walls up to the shooting towers inside our base adjacent to our keep. Our biggest concern is running out of ammo and radiated zombies, especially since we've modded our zombies to be bullet sponges anywhere but head-shots.
The great thing is we get a ton of XP doing this. Eventually we will install turrets that have blinders at the ends of the corridors so that they only shoot at zombies that have crossed a certain point on the corridor. The flanking firing arcs tend to knock the zombies off the corridors, forcing them to find their way back to the ramps.
