>>43507361If we're talking about "mythical creatures capable of causing catacclysmic disasters", then "fortifying" the building will do as much good as fortifying the train; close to nothing.
Confining this type of events to tiny ass spaces is really dangerous no matter how you put it; have some actual strong shits battling and they'll end up crashing the fight 5 floors down easily, and that's just hoping it doesn't compromise the structural stability of the entire building.
So if we are going to be pedantic about hypotetical realistic battles, then do it right: only open areas and big stadiums at ground level would be relatively safe for elite fights.