Today I took a look at the bug: "zombies wont attack hatches nor climb ladders if covered with a hatch".
It turns out that pathing did not make downward connections on ladders, so they could not path down the typical 1m mineshaft with a ladder down it, so they would get over your head and dig. That is fixed and when covered by an obstacle like a hatch, they will go to it and dig.
On the climbing ladders side, they now check if blocked above and target that block, so hatches get attacked.
Changes:
Fixed SDTD-10733 zombies wont attack hatches nor climb ladders if covered with a hatch (fixed ladder path connections down and connections go fully to the top. changed digging to trigger on blocked down paths. added blocked up check when climbing).
Added pathing will cross 1m ladder gaps.