"Mountains can only be crossed by units with mountainstride."

Bug is units without mountainstride can't even attack figures on mountains.

My guys are standing right next to his building on a mountain and they just get to stare at it.

This is the way it is supposed to work. In general you can't attack spaces that you can't move to. You can attack them only if they attack you then you can "Battle Back"

When Figure A melees Figure B, Figure B is given the chance to attack Figure A back even if Figure B normally wouldn't be able to move there. It can only Battle back if it is undimmed though.
For example a Driftwood Golem in an ocean attacks an Axe Captain. Normally the Axe Captain can't attack the Golem because it can't move to an ocean. But since the Golem attacked first the Axe Captain is allowed to retaliate.

Then the cost of that enchantment needs to be increased quite a bit. It's only 1 domain and 2 flux (maybe 3? I'm at work and can't look)
With Volcanos and the Trickster you can create all kinds of mountains and just become near invincible. The enchant to make corpses have fear requires two domain and 3 (2?)flux. That's not quite as powerful, it doesn't make figures unable to be attacked.

I think the enchantment is to powerful if it's going to remain the way it is, even with an increased cost. If you play any domain other than mountain folk, there's only 1 or 2 things you could even try to do against a building on a mountain.
Off the top of my head
With sylvan there's a few archer units you could use.
With Dark Legion, I can't think of anything you could do. I don't believe they have archer units? Could use that one enchantment, that negates a building giving control.

I'll have to look when I get home and see what options the other domains have.
I'll also make a deck around it and we'll have to test it out one night.

Maybe you are right. I didn't actually think about the fact that building on mountains would be really hard to get at.
Yeah please make a deck I'd like to see.

Actually DL has quite a few things:
skull catapult, that Fiend, Fire beetle, carrion feeder, Barbed necrath, octopus and Foul Obelisk

How would carrion feeder do damage to it?

Undead Octopus and Foul Obelisk only half count, they're very very situational.

carrion feeder is aloft so that should be able to cross mountains still. I should change the text of the card to make this more clear.

Ah, yes I missed that Aloft part.

And what about units with hover?

Actually now I remember the intention was that neither aloft nor hover can cross these spaces. But the I didn't mean to protect buildings like that. so what if it was:

Mountains without buildings can only be crossed by figures with mountainstride. Only Buildings that spawn in mountains can be placed in mountains.

But I can hit my building with volcano. What then?