Mineholes are one of the worse resources to lose at anytime in the world.
Anything can smash a minehole~ I think mineholes need to be made a little more resilient.
Adding a 2 step process of building a mine hole.
Using Knarr mechanics (adding the sail) the minehole should have the ladder added in the 2ed step.
The first part is building the hole, would cost the normal resources.
Once the hole is built anyone can enter the hole, but going down the hole without a ladder will result in a major injury and noway backup other then to hearth.
Adding the "minehole rope ladder" will allow the minehole to act as it currently does. The rope ladder once added should be unremovable.
This option would allow players to prepare their lower mine levels without having to deal with raiders mining into their tunnels and smashing their minehole('s) immendently.
The extra damage step also stops players from just using the minehole without ladder as a oneway gate.
Lastly blocking bellow should be possible still if the ladder has not been added, I think.
Additionally~ to destroy a mine you would have to fill it with boulders