Well I'd suggest alternate dimension, we keep it confined to the Tower whilst we try to figure out (who we are, what we're there for).

We're the Discardables, the unwanted of society. Those few that show a potential Quirk are taken immediately to the Tower...Becoming Potentials.
Meaning they might be of use regarding the safekeeping of the city or the protecting and or powering of the fence. 
Society on the outside of the Tower is a world of energy and electricity. 

The world has had a cataclysm event of sorts, how it exactly happened no one knows, but people fucked up and disturbed or released something they should have left alone. 
Those that survived the cataclysm live inside 'safe cities' protected by a large electric/magnetic fence that keep the elements and potential other dangers out. This fence acts like a dome over the cities.
The city's themselves are a mix of remnants of the past married with high tech skyscrapers.
(Think of a world where the sun's light can't penetrate the storms or the fence, simply lighting up the sky in weird flashes and changes of colour like a dimmed moodlamp)
Due to the conditions being what they are Cities usually have a large watersource nearby or they are build on top of underground watersources.