I thought of some dangerous place like in Inverted Fate there was an Aquarius Fort in Waterfall, so I thought it can't be a fort cause that is just clearly taken and unoriginal now. How about a castle? Would be stupid, there is only one king in the underground. A mansion of some important persona in the underground? Could work, would have guards in there if was so important after all.
So it escalated.
Although the Core will be different in this AU Take. Not much people are gonna appear there and it's gonna be darker there. People are scared to go there. Some areas even collapses. But of course Core still works, it's not horrortale, the ceiling can collapse however and light may not work. I don't mean the whole place will be dark, but some parts. Probably most since it's a place not to enter and rather not worked on anymore. Alphys probably does it in the original? Anyways won't be here. Gaster made the Core as a whole and it's too complicated for the lizard.