How do I properly place a mine?

Solution 1:

Here's the footprint of the mine's placement, per Banished-Wiki:

Legend:

• Tile - Green Arrow: The front of the mine. You can place a path here.
• Tile - Green Space: This land must be relatively flat.
• Tile - Brown Space: This land must be part of the mountainside.

As of this posting there is no terrain editor and the game is pretty good about telling you where you can and place structures (as you've indicated), so it may just be the case that you won't be able to use that location. Is this a lone, done-shaped mountain you've found?

In my experience, I've had trouble with these (unless they're sufficiently large) as well as mountain ridges with a very convex outline. Try finding a relatively linear mountainside instead?