Describe off the top of your head the external structure of the Royal Ontario Museum located in Toronto, Ontario.

If you were going to describe the Royal Ontario Museum off the top of your head, that would mean you do so without doing research. If you know a lot about architecture, you could apply that, but otherwise, you would be giving impressions of the building, describing what it looks like, what it seems like, and what feelings it gives you.
If I did that, the first thing that I'd say would be a list of words or phrases: jagged. Shiny. Geometrical. Huh? Modern. Probably expensive.
If I had to pull that together into a more rational and coherent response, I'd end up with something like the following:
The Royal Ontario Museum certainly makes an impression. It doesn't look like a museum, but more like the entire thing is a work of art in itself—a work of modern art. The museum's exterior is vivid, geometrical, jagged, and austere. The sharp angles aren't very welcoming, but they do draw the eye and claim space. It kind of looks like an alien spaceship crashed to earth, and half of it is sunk in the ground. It looks like it was designed to stand out and call attention to itself, rather than be welcoming.
https://www.thecanadianencyclopedia.ca/en/article/royal-ontario-museum