This is the Belvedere Museum in Vienna. It was originally built as a palace, of course, but it never got used much. There were just too many other palaces in Vienna. Eventually, after 50 years of sporadic use, Maria Therese transferred the Imperial Picture Gallery to the Belvedere and it's been an art museum ever since.

I count (at most) twenty-five people in that picture. That strikes me as unreal. Beautiful day. Beautiful building (not counting what's inside).
The entrance is on the other side, and it's more crowded there. But not very. May is a pretty good time to visit Vienna.
It was built for Prince Eugen of Savoy, the 17th/18th century Habsburg military hero. His expansive library was acquired by the state as well and became one of the core collections of the imperial library in the Hofburg. One of the more colorful characters of early modern Europe.
You should see the interiors.
https://youtu.be/8zykAbeXx7A?si=rQCnwP29OHkZvYCi&t=99
They flog the Klimts and whatever temporary exibits are on, but the collection goes back to some really great stuff from the Medieval period. When I was there in the Spring pre-COVID I noticed a late closing one day and was there from afternoon to after it got dark out. Just checked - no late hours now. Really not to be missed if you are there.
I had a guest last weekend here in Brooklyn and we went to the Met on Saturday, a late closing day. Three hour wait (on an app) after doing the hashtag right after entering to get into the second last day of the Fashion Insitute show. Just made it thorugh. And then to Titanique at 9 pm. That's how to do your efficient tourister scheduling, people.
At least one of the shrubberies is annoying me greatly, being ever so slightly askew; but I shall visit. Actually it piques my interest.