Over various posts,I've heard about "doors". What are these,and where are they?

The most interesting door is up by the ruins. There is a "hole" in the earth with a ramp and a closed door at the end.
Admittedly we know nobody that has open this door or some of the other doors of the buildings in the town.

There is a door on the castle, one on the resort, and one on the temple in the ruins besides the pit door.