I am not sure what is causing this error in pathfinding, where one of my colony's extensions seems to periodically decide it can't figure out how to use the main road, and goes into and out of siege mode. I'm thinking I should make a dedicated tunnel where only the threat level 0.8 zombies can actually spawn in, so I can set the one resident to a slinger guard and have them take care of it.
But how does the pathfinding work, so I know how to avoid this in the future, if not remedy it now without a tunnel bore?