The husk seems to follow a simple A* pathfinding that take into account the hit points of the structures that have to destroy has “cost”. The map is based on “cells” and the more HP hitpoints you can stack in that cell, the more undesirable you can make it for husk to use. A |\ shape is generally enough, and can be stack with |\|\|\ made of wood. A palisade like that is cheap and fast to build.
The husk are not omniscient, people on the internet says that they only consider routes 3 tiles away. So you can’t cover your entire base with iron level 3 except from a hole and expect the husk to use that hole. On the contrary. Put the dead tunnels in the straigh forward routes the husk will want to use anyway, and block secondary routes so you only have to defend the tunnel.
For the reasons above, If the husk are going to attack from the south and the north, you need 2 death tunnels. If you have a defense and the place is really bad, with the mobs being able to attack from every direction, good palisades can reduce that to 4 directions, and sometimes to 3 death tunnels.
Edit: the |\ shape can be made stronger with a floor that include mid wall, but that takes more time to build and I think is unnecessary.
Screenshot in case my ascii art is too obscure:
