Multi-Floor Base Design
Zombies don't climb. Anything above the ground floor is invisible to them unless they can reach it. That makes vertical base design the single biggest survivability upgrade after a wall material change.
Floor 0 (ground) — defensive
Walls, doors, your kill funnel, your fridge bank if you can power it. Everything here is replaceable; assume it gets hit.
Floor 1+ — living
Beds, crafting stations, storage of everything important. If the ground floor falls, you bug out via sheet rope and rebuild downstairs.
Stair removal
The big trick: remove your own staircase once you're upstairs, build a new one as a temporary access ladder, and you have a completely unreachable floor. The planner's threat overlay assumes zombies can't climb, so above-ground tiles never light up red.