(I posted something earlier and was asked to edit it. I'm new here so … learning is painful, what can I say)
I'm Derek B. Miller. I'm the author of Norwegian by Night, The Girl in Green, and American by Day. I have a forthcoming sci-fi novel called RADIO LIFE being released in autumn, 2020. I am already prepping for the sequel, and you folks are the go-to source for great ideas.
Set-up: It's roughly 400 years from now. In about 2100 (to keep dates simple), the world population was wiped out, mostly from countries acting badly. Nano-tech was used to deliver bio weapons; nukes were used in a few cases; the vast majority of the world pop was killed off. Many animals, of course, but they have largely recovered.
The question is quite focused now: What condition would we likely find the Guggenheim Museum in Bilbao, Spain?
Keeping in mind climate change (natural and worsening between now and 2100); water levels changing; acidity; natural plant growth in formerly populated areas (etc.), animals wandering about it.
FINAL THOUGHTS: 1. You have time between now (2020) and the collapse (2100) to add some tech and engineering solutions to try and preserve the building — not in anticipation of a war, but certainly in anticipation of a worsening climate 2. It is OK for people to have either stayed there for shelter (and therefore continued to tend to it) or discovered it a hundred or two hundred years later and decided to stick around. 3. THERE IS NO INTERNET ANYMORE. The world went Digital, the digital failed, and now no one knows anything. So no using hi-tech solutions AFTER 2100.