How do I go up staircases?

A lot of maps seem to have stairways on them. As far as I can tell, I can't walk up them, but sometimes I see NPCs running around up there.

Is there a way I can have my troops go up staircases?

Okay, it turns out that you need to set the elevation slider to be at the exact level you want to move to. I'd forgotten it was there and kept hitting the button that shows/hides the rooftops. It's a little fiddly since sometimes it changes elevation levels on you, or you can't select a guy on ground level when viewing higher levels, but it is manageable once you know what you're doing.

I've circled the slider in the screenshots below. Note that you can also use the TAB key to cycle through the elevation levels.