For the sake of info, weighing a couple of womens sized bikes in our garage:

Trek 7.2 weighs 33 pounds, with rack, basket, and heavy cable lock.
Early 90s entry level road bike with mountain bike style handlebars weighs 27 pounds.

In terms of maneuverability up and down stairs, my main preference is the old fashioned trapezoidal frame, because I can sling the whole bike over my shoulder. Those frames are getting harder to find except on used bikes. The more compact frames on newer bikes seem to offer nowhere to grab hold of them, so if you're looking at bikes, I think it's worth actually trying to pick one up and carry it.

In terms of "some assembly required," an honest to goodness classic road bike with upright bars and single speed drive train might be ideal. You could choose a gear ratio that gets you up the hills, at the expense of top speed. That's what I rode to work when I had a university teaching job.