Bombard thinks the Underdark isn't an exterior area.

[Huelander]

While it is arguably correct to say the Underdark has no true exterior areas; it seems strange not to be able to use siege weapons in caverns as large as those known in the Underdark. And effectively making them surface-exclusive.

Now, I might be wrong. But I tried it in a bunch of underdark areas and none of them would function. Around the areas of Darkfalls and the Outpost.

I don't know how the script checks for the area's eligibility. But I would like to file a report for the team to consider this matter!

[Batrachophrenoboocosmomachia]

Quick fix, in the pipeline.