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Old Mineshaft [VN]

Place the tunnel somewhere above the map and the entrance where you want them. Don't place the tunnels in the no man's land around the map. If you do so, explosions will ignore walls. Use the layers to hide, for example the ceiling, for a better view inside. Make sure you diabled vertical mode and surface snapping in the editor before placing the tunnel.

To enter/leave the tunnel, you need to interact via mousewheel with the minecart in front of the doors.
When you enter the tunnel, I used 'setAperature' to darken the player's view a bit down. The setting will be set back to -1 if you leave the tunnel.

Enjoy!

When you place AI makes sure you disable their pathing, so they don't walk around.
To disable the pathing for the entire group, use the following code in the init of one member:
{
_x disableAI "PATH";
} forEach units group this;

If you want the AI to stare in one direction with restricted aim use
this disableAI "MOVE";

If you use the ACE mod you can also add an interaction point to the minecart instead of the mousewheel action. For this there are comments placed where you can find the scripts for the init of the minecart.