TL;DR: Time-to-reach-you. It's all how long it takes to reach you. How long it takes to block, how long it takes to walk a path, how long it takes to ascend/descend as needed. Doors count as half HP (To represent the potential to be open or closed).
Shortest route, in time taken, is king. But if a zombie approaches from another direction it may decide beating through the wall there is faster than walking around to whatever opening some other zombies made; It may take them 20 seconds to walk around, but only 18 to beat through the wall and come from that direction.
For this reason, mazes of most types arent really feasible; they'll just choose to bash through. It's possible to use a maze like structure but it requires great care and player positioning to guide them through.
As for traps, Zombies do not recognize them as traps; They recognize them as blocks in the world to be navigated or destroyed. This includes spikes- If you build a 2 tall pile of spik...
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