Have you actually looked out of the fortress? That place is completely unsiegeable.
It is perfectly siegeable. Due to its location, it's in fact ridiculously easy to lay siege. Remote locations make for easily defended spots because enemy approaches can only come from limited directions. The same paths your food comes from. Besieging a mountain fortress is not difficult at all, it just takes time. You can't take it by force of course, but who says you can't just camp a sufficient number of people at whatever orads that lead up to it and cut it off from reinforcements and supplies. Send some patrols out to look for hidden paths. Anything big enough to supply a fortress like that is easy to find if you have compentent scouts, and any paths compentent scouts can't find aren't big enough to supply the fortress. Now all you have to do is wait.
Granted, depending on how big the stockpiled supplies are, that wait can be measured in years (and a mountain can also be hollowed out to make room for storage), but the point of a remote fortress is not power projection, it's keeping whatever is in there safe. And since an Inquisition that's safe inside its fortress can't inquire much, the guys laying the siege win.